Hi - I am fairly new here.
I know what you mean that it can build and build!!!
I feel you. BM doesn't like anything I do especially if it's nice things. I always wondered why. She's gotten worse since DH and I had DS. She doesn't mind using me as a babysitter but she found out I took my DD and DSD to buy shoes and flipped out. (So, next time I'll drop off DSD at the curb and go buy my kid shoes, yeah, no.)
I did learn to never send anything home with DSD that involves me. One time we made brownies and BM was there to pick up DSD early so DSD didn't get a chance to have any, I sent some home and DH got a complaint email that DSD got the brownies all over her car. Yes, that was my fault.
I think it hurts when someone you've never done anything to hates you for no apparent reason. It doesn't make sense. Best advice I can give you? Grow thick skin and keep being good to your SK's.
I'm sorry you're having a difficult time after all these years.