Everybody is different but I had sex on the 16th and my last period was the 6 th of August. Than had sex on the 24th. I concevied around the 20th. I also have a 28 day cycle. Good Luck
Have you had an ultrasound during the pregnancy, especially one before your 12th week? Did they give you any information from the ultrasound, such as a gestational age (the date that starts with the estimated first day of your last period) or a due date?
With that small of a window there really is no way to safely say who fathered your child, a DNA test will be your best option Hun best of luck! (If your cycles are a regular 28 days then its likely to be guy #1 but there is always a possibility of late ovulation)