Based on lmp (may12), given may26 as doc. had unprotected sex w/t guy 1 may21. and1 nitestand w/t guy 2 on may26 but used condom. most likely father?

Can't tell reliably: The encounters are to close in timing to be able to give a reliable answer. Ovulation will typically occur two weeks after the first day of the period, but that is in the idealized 28 day cycle. If your cycles are shorter, ovulation may occur a little earlier. Having used a condom with guy 2 would tend to reduce the chances that he is the father but certainly does not eliminate that possibility.