I am not a medical professional, but would assume that since you had a period in between both men, that only the man who you had sex with AFTER your last period could be the father.
The words "It sounds" got cut off in what I wrote above. It sounds like, based on your last period, conception was in mid July, but do you have any other confirming evidence, is what I was asking.
Well my first ultrasound was done when I was twenty weeks and she was measuring on schedule. I was having this doubt earlier today because i had an appointment today and she measured my belly. i'm measuring at 30 and I kind of felt I like I am a little big to only be 7 months. My doctor told me that it's just probably a lot of fluid around her. I felt and somewhat already knew that it was my first childs father baby.