Have you tried talking to her about it. Maybe take her to the doctor and see what they say.
If you can afford it (your insurance covers it) try talking to a therapist i.e. a psychologist with your daughter. I guess it goes without saying that psychologists who have experience with children are probably the best choice.
In the meantime, or if you can't get professional help, try to find out why she wants diapers. Don't pry too much and reassure her that whatever the answer, you love her and deeply care for her. Make It clear you are aware she probably doesn't wet the bed and that she should feel safe telling you of any other concerns she has.
What you did previously, threatening to shame her, will make her clam up about the diaper wearing. It then becomes a secret that she may or may not pursue in private and that presents a bigger problem and possibly there's little hope of resolving it.
Finally, the therapist isn't there just to coax an answer out of her. They will also try to help you resolve your feelings towards this behavior. In their eyes, it's about giving you both a bit more perspective and hopefully mutual understanding and eventually setting healthy boundaries when it comes to the behavior in question.
The crux of it is, make sure your daughter feels supported and able to talk about difficult topics with you. Avoid shaming, and allow for conversation. Parenting is hard. Good luck.
You're not alone. My 6yo step-daughter showed signs of regression and interest in wearing diapers/pull-ups before she turned 5. It is not uncommon for potty trained kids to suddenly want to wear diapers. Trying to stop this behavior through criticizing, shaming, or depriving it will only reinforce it, and you will harm your relationship with her.
This is my story for how my wife and I researched this behavior and what we did in order to maintain harmony and positive emotional growth with ours.
She has ADD, is on the spectrum, but high-functioning. Her regression was introduced through bed wetting due to various stressors. Mom and bio-father splitting up,(never married) lost their home due to foreclosure—neglect of her dad. Her bio-father has mental illnesses, alcoholic, gambler, irresponsible. This kept my GF trapped as primary caregiver and housekeeper while he worked and pissed away their income to alcohol and gambling. She had to break away for her daughter and her own welfare. They were more or less done with each other but stayed together as they moved in with relatives to maintain the image of mommy and daddy for their daughter. She wore pull-ups at night for convenience.
She was very fearful of being left in a room alone, and couldn’t go potty by herself. If I were in the bathroom I’d later find her scared in a corner under blankets and she’d have peed her pants. Other times she’d deliberately pee her pants in front of me. I’d calmly say, ‘that’s a silly thing to do. why did you pee your pants?’ And she’d say ‘because I want a pull-up’. I’d tell her ‘use the potty, those pull-ups are for nighttime' Some days she’d have held her pee all day. At bedtime we’d tell her to go potty, she would go(or so we thought) we’d get her into the pull-up, she’d stare off into space, giggle, and declare “I’m pee-peeing my pull-up...” completely soaking it. We’d get a little serious and tell her not to do that, you’re supposed to go in the potty, take it off and throw it away, then stick her in a dry one. Some mornings she would wake up dry and before we could tell her to get dressed she’d use it instead of taking it off and using the toilet. Other times I’d get her dressed and later that day notice a bulge in her pants. We wanted to take the pull-ups out of the equation when she stopped bedwetting or nighttime potty train her but that was impossible. She is a heavy sleeper, can't be woken easily, and requires children's brand melatonin to get to sleep.
We knew how common regression can be influenced by stresses, major life changes, and she checked the list. Children on the spectrum do have greater difficulty adjusting to change and they will harbor a deeper rutted resentment than neurotypical when forced out of their comfort zones. When she asked to wear them, we calmly discussed it with her, asked if this is something that she likes, and she told us she likes wearing and peeing in them. We tried explaining the social problems this could cause for her with friends in words she could understand. Knowing that if she is deprived and forbidden to having it we would see other un-desirable behaviors, her feeling isolated, and ashamed of herself. Stealing diapers, buying them in secret, or contacting strangers on the internet when she's older.
In our research we read many stories of adults that had this diaper obsession as kids who were met with shame, rejection, and other negative responses from their parents, and it only caused emotionally damage and reinforced their desires. We think that if we rejected, criticized, or shamed her it would prolong the desire and harm our relationship with her. Then there is the possibility if we just indulge and allow it, this could just be a phase she might grow out of.
Soooooo... We now allow her to wear diapers and pull-ups during the daytime when she wants them.
Allowing it became a privilege. She knows they cost money and can be taken away for bad behavior. They have helped sensory overloads or meltdown, when she can’t deal with reality, and needs the comfort of a diaper while cuddling with me or mom like a baby. She is much more adaptable to new routines, and learning new things has been easier and helped reduce the amount of nervous energy she often displays. She fully understands it’s her secret, and she could be made fun by other kids. She mostly wears them at home. It took time, but she understand the hygiene upkeep involved in the ritual of changing and getting cleaned up. Initially it was that she change herself, but she prefers that I change her. She knows what she wants, when its inappropriate to wear them, and some days she’ll choose panties even if there are no day plans where panties are the better choice. She doesn’t wet her pants on purpose anymore and it's not like she is un-potty trained. She occasionally has accidents but that's normal for a kid on the edge of special needs.
We accept her as she is and love her unconditionally. Diapers and pull-ups had become her imprinted transitional object through no fault of herself. She experienced many stresses that drove her to desire the safety, security, simplicity, and routine permanence she had from a different time in her life. During that time she was still in diapers, and she seeks to have that as a means to cope with what she’s been through and could still be going through. Pull-ups and diapers are now just recreation, and sensory therapy objects for her. It just happens to be on her body, and she likes the feeling of peeing in it... She couldn’t be happier, and my bond with her grew very rapidly since our choice to allow it.
We feel that since allowing her pull-ups and diapers in the daytime has been a small positive contribution to her growth and well-being. She is much more loving and is very open to talking to us about her feelings, and doesn’t clam up about something that’s bothering her. I never thought that in the short amount of time I had been parenting her she’d tell me that she loves me, frequently asks me when Mommy and I are getting married. Expresses that she really wants me to be her step-dad. When will mommy and I make her a baby brother or sister.
We can safely say that allowing it in the way we have was probably the best resolution. "This makes you happy? And you feel good in them? And you understand that if your friends learn about this, they will probably make fun of you? Ok, whatever floats your boat... we love you and support you." Her mother and I would much rather have a very strong loving relationship with our kid early on, than having a socially distant kid full of resentment, shame, emotional traumas, likely closed off to discussing her problems with us.
If you can learn to be ok with it, accepting her as she is, and her understanding that it does not disgust you, and she feels safe with you about this. You may begin to see a completely different personality that can be nurtured at a very different pace.