My bf and i have sex all the time and he ejaculates in me all the time. We were on a break for a little while?

and i slept with another man unprotected for literally 10 seconds, no he did not get to climax. But now I'm pregnant and the docotor said I conceived 3 days after I slept with him. Im not sure if its his or my bf because we got back together the next day but I dont remember having sex with him. And i have no idea on what to do.. help