I'd suggest you wait until you know for sure you are pregnant, and then get an ultrasound when it is 7 weeks from the first day of your last period. Then count back from the due date they give you. Be sure the doctor knows your first day of your last period is not reliable for determining the length of the pregnancy because your cycles are irregular, ask for a due date based on the information in the ultrasound alone. BUT all of this discussion could well be moot, because your chances of pregnancy in this situation are pretty slim.
(And, incidentally, don't have sex with two guys in one cycle again after this.)