Boyfriends little girl.

Okay long story,

Me and my boyfriend have been seeing each other for a while now within this time frame I’ve had to press assault charges on his ex(also baby mom) because she attacked me while I’m pregnant. Sometimes he has to leave his daughter with me when he goes to work if he has her those days usually we have her thurs-sun well in those times I’ve had to watch her I’ve noticed sometimes she’s really irritated around her vagina and its looked like it’s been open it’s not like that every time. Me and his sister were talking and she has said she’s also noticed it but we can’t assume anything because she can’t talk yet. Here recently she has been waking up screaming like she’s in pain in the middle of the night and we have noticed too that she’s been playing with herself... it could be an infection or something I’m just worried because I know her mom is sleeping with random guys and letting them stay the night as well. What should I do ???? I’m really stuck on this one.