What has gotten in to my 3year old pit bull?

My dog Selma is part pit and part boxer. She is 3years old. We have an 11 1/2montg old part pit part jack russel pup. The pups name is Trixie. Trixie is in heat for the 1st time and Selma has tried to start a fight with Trixie. It happened today and yesterday. What is going on with Selma??? Both dogs r treated equally. Please help!!!

dog in heat in her house

Its probably bc its another female and she's in heat, when i had two females they would almost kill each other each time the other went in heat. One time they went into heat together and it was horrible, they fought while everyone in the house was at work and school and when we got home there was dry blood and scratches everywhere on our wooden porch. Its prob. a territorial thing and best to keep them apart until its over