I was going to add, if you have total clockwork cycles with periods every 28 days without fail, for months and years and never any difference even by a day, the encounter with the second guy is more reasonable to think would produce a pregnancy than the times with the first guy. But, most women don't have total clockwork cycles. And, anyway, we haven't even established that you are pregnant.
Are you saying *if* you got pregnant from the sex, who would be the dad? That's kind of putting the cart before the horse, your chances of pregnancy are only about 20% in a given month if you don't use protection. (Hate to say it but if the sex was unprotected, you have a better chance to get an STD. You might want to get swabbed.)
Are your menstrual months super regular, such that your periods come exactly every 28 days month after month without variation? Or do they come on whatever random day? If the latter, it's not possible to guess who the dad might be from when you were expecting your period. (If you're pregnant in the first place.) I guess I would advise that you wait to see if your period comes. If it does not, take an early-results pregnancy test. If it's positive, write to me and then see your doctor. I'll give you some info for what kind of imaging to ask for and when, so you might be able to guess from it which guy is the dad.
Good luck, my advice is, don't build a huge hill of worry until you have to.