Probably because of trauma. She has screamed at you before when you hadn't done anything wrong? (mine did....)

Maybe try to talk to her. If she is receptive.... (mine was not, but told all the family and friends that I was a ''monster'' And they believed her. I hope your mom isn't a covert Narcissist like mine was.

The step maternal unit is a Sociopath with Normotic features. UGH