possible to have gotten pregnant from that one time...?

I was on and off with someone(my ex) for 2 years. During our off time, I was talking to someone.

Well, April 4th-8th I had a normal (3-4 day) period, my periods range from 30-32 days. I had sex with the guy I was talking to ONCE in the month of April on the 13th in which he had pulled out (never went inside of me). Well my ex begged me to take him back on April 15th, April 16th- present he has been the only person I have had sex with. **had no idea he was going to beg me to take him back, we had been done for 2 months or so** Well my ex never pulls out of me when we have sex (at least 3-4 times a day), sorry TMI. Well I took 2 pregnancy tests May 6th to be on the safe side, they both came back negative. On May 8th I got a period that lasted 3-4 days (before now, i never questioned this period). Well continuing to have sex, I never got my period in June (pregnancy test June 12th confirmed I was pregnant).

I went to the Dr. at 8, 14, 18, and 21 weeks pregnant. All confirmed a due date of Feb. 12th (from LMP) & Feb. 15th (based on ultrasound), never once have they been any different. She measures perfectly to be due the 12th-15th.

I was wondering if it was possible to have conceived and gotten pregnant from that one time on April 13th? Or who you believed the father could be!!

Thanks!!