Chance for me being the father?

my ex-girlfriend is 27 weeks pregnant. i had with her on these days july 9,10,14,15, 19(maybe) and the 21. and she had with a second guy on the 24 of july. is it possible i can be the father or not. what are the chances? is it high or low? can doctor pinpoint the day she concieve? is the ultrasound accuate? right now she is having a boy. i want to know if i should just move on and forget about but something is holding me back cuz i don't want to have a baby out there who is mines that i don't even know don't have a father ya know. i the type of guy who will be there for my kid. not a dead beat father or don't care. i was raise as a family oritented person. so what should i do. any advice. to confuse about this whole thing. i had unprotected with her during those days i post. she told me she is due in march or early apirl. i remember after we had on july 21 on july 22 she had ovulation pains(feelin hot like she had a fever but she didn't,nausa,pain in the lower abomen.