Unprotected sex while on birth control, last day of period

I am really worried- my boyfriend and I broke up for a couple weeks in April and I slept with another guy. I started my period on the first of April and finished on the 7th of April, which is the same day I slept with the other guy. During the next week, my boyfriend and I had sex several times. I was on birth control but missed a couple of days during that week. I found out the first week of May that I was pregnant and I'm wondering what the likelihood of the first guy being the father is. I am scared to death that it's him, but don't know if it's really very possible (since I was on birth control and had regular periods) for me to have gotten pregnant on the last day of my period...on top of that, the other guy had a lot of trouble "getting it up" and was inside of me for only a couple of minutes. I'm honestly not sure he even ejaculated.
I'm due in a month and a half and the conception calenders all say I conceived on the 14th of April...which would be the boyfriend...can anyone help?