I wonder if she has been hearing any of this, from the way you describe the language and behavior, so I would check out whats happning when shes not with you, has she older siblings, how does she get on with your new BF .there seem to be a lot of factors going on here , good Luck
Yes, I think the changes have something to do with this. Many kids hear a 'voice' in their head that sounds like themselves . . . don't you hear your own sometimes? That it is saying bad things makes me sad for your little girl. She does sound like she is distressed emotionally. Lots of big changes for her in a short time.
I am thinking that the new baby coming is a biggie, all the new people--- husband's girlfriend, your husband, etc. Hopefully you and her dad try to remain on good terms for some consistency.
The only other thought I have and it is not a good one is that something is going on over at her dad's house. Hateful language, negative talk, the 'boys parts' and F word, etc. I'd investigate further with her about that.
But overall, do you think your daughter could use a neutral party to speak with such as a therapist specializing in kids?
Also, if she is 6, she's in elementary school. Talk to your school counselor---- our school has various small group support groups for kids going through life changes at home like divorce. The counselor can be a great help to you with her I would think.
Anyway, keep your eye on this and consider the counselor. I'd try your very best to not have much more change with what you can control (on your end, no big moves, no new baby from you, etc.). good luck