Pregnant, had two different partners, who could be the father?

I have a pretty normal peroid i usually get it the first week of the month. Im pregnant, my last peroid was the begining of january. I had sex with this guy a few weeks after, and then on january 28th i had sex with my ex boyfriend. zIt was about two weeks apart, and i really didn't start to show have signs of pregnancy until after i had sex with my ex, however i still know theres a chance it could be the other guys. At my first doctors appointment i was told my due date would be October 8th, well i went to my first ultrasound may 23rd because i didn't find out i was pregnant until march.Aanyway the woman told me because my baby was small as was her heart i was probably 18 weeks pregnant. i've been staring at my calendar for at least an hour now trying to figure this all out. i know i won't know for sure exactly who the father is until i get a dna test which im deffinatly getting. but i wanted an outsiders opinion.