Chances of Pregnancy?

So I have a new fwb. Last night we got very carried away. He ended up finishing in me. He was a bit worried about me getting pregnant, I told him I'm 3 days late already. My period has been off for the last 6 months as is usually off by a week. But we're are fast approaching the end of the month, so if I don't start, obviously testing (it would be someone elses if positive), but if I continue not to start and its negative, is there a chance I could conceive a child with this guy?