We got back together, but she hooked up with other guy during our break up

I need help desperately... I need some help figuring out when my girlfriend of 13 soon to be 14 years concieved.we had a breakup on may 8th 2016,and she had sexual intercourse on the 8th and 9th of May with her new man and the man ejacukated in her.the last time we had sex was sometime within the last week of April and I pulled out.we believe her period began around aroung April 24.her periods come and she may bleed for a couple days and be done.we r struggling to figure this out as we r back together and she wants me to be the father but things just don't add up to me being the one who fathered her baby.we have been together so long and we have a12 and 9yr old and I'm not sure if I can accept this baby into our home and be a part of raising it.we have discussed adoption but unfortunately the mom cannot bring herself to doing this which I can completely understand her feelings as I could never allow one of my children to be adopted either.im told to pay attention to the ultrasound picture which was on July 1st where it said she was 9 weeks and 5 days pregnant.according to prenatal advice she concieved between may 8 9 10.this would make the baby not mine.although I'm told there's still a chance I am baby's daddy.how?could someone please help me!