delayed finish?

So so and I had a few drinks the other night, had sex and when he went to finish on my stomach, we realized after looking that it never actually came out. Two minutes later after he got up it came out and all over his leg. 
So confused, we thought we lost where it went, but this was not post cum, it was like, the whole thing. Never heard of this happening before. It's like it got backed up or something. Anyone familiar?