I'm only 17 and have been with my boyfriend for 2 years. 6 months into the relationship I had no interest in having sex with him. ( I actually had no feeling having sex the whole time, but because the idea of us having intercourse turned me on I was excited to have sex. I still haven't admitted I had actually never felt any pleasure during sex to my boyfriend. I actually lied to my boyfriend telling him that I no longer felt anything during sex when there was no feelings in the first place. I did this in fear of hurting his feelings. I had been faking moans orgasms etc the whole time to make him happy. Although I am not in your shoes and can't say you are in the same boat I feel are situations could be similar.
Try to make it more about her. Give a little take a little. Hit puberty at 8 and have been sexual since. Started masturbating watching porn etc at a young age so I expected alot from sex. Therefore plain old sex got boring quick. Try to explore new things, things you wouldn't think of usually. Try to lean more towards forplay to turn her on. I need to be extremely turned on to feel anything during sex. I'm very thank you to my boyfriend who now jumps through hoops some days to turn me on and help me reach orgasm. In return I want to please him the same and give him sex.
It could be emotions. Women are emotional creatures and if she feels disconnect from you it could be affecting her. This may or may not be your fault. If she was abused, this could magnify small things to her to cause problems in your marriage. You can do a soul search and/or ask her if you did or acted in any way that bothers her. She could have shoved it down so far that it takes a few days for her to remember it.