Im pregnant was on the pill&two guys involved??help>>

I had my last period either aug 4 or 6th but i am pretty sure it was the 6th.During this month i was back&forth w. my ex&seen another guy also. The doctors say that i had conceived the last week of aug,but i last slept w. the other guy the 25 of aug.& i hadnt went to the docs till dec and thats when i had found out that i was pregnant. I had waited so long bc i started the pill in aug when ihad gotten my period& i had my period for 7 days than like two weeks later i was spottin so i finished my pills&than had not gotten any refills.During the last week of aug i had a bladder infection&had gotten on antibiotics the 27th&didnt sleep w. any one.But i was constantly back&forth w. my ex.So i can be too sure when i concieved&who the dad is. I guess the best bet in an paternity test when the baby is born.& my due date they said from my first ultrasound is may 28