you had sex like 3 weeks apart so when you have your first ultrasound you should kno your due date and count it up
It sounds like you got your period the first time you had sex, It most likly would be the second guy. But like what Kate said, you can get an ultra sound and see around when you got pregnet. If your still wondering who the father is after that, you can get a paternity test after the baby is born! good luck! :)
When you go for your ultra sound you should be able to tell you how far along you are good LUck
the ONLY way you'll know who the father is is too get a paternity test done.