Why is my female dog trying to mate with my other female dog?

Ok, so my female jack chi is trying to breed with my female chi who is in heat. I don't know why the female chi(Mimi) has been in heat before and my female jack chi(Snowbella) has never done this before. I did get a male week old chi puppy a about last week. I am just woundering why they are doging this. I know it is not for domonents Becasue she is doing the same thing as the males do with her. She is a vary doment dog but she is mounting her and holding her down, and stuff. should i be warried?