Had he perhaps ejaculated in you? If this is the case, when you ejaculate/orgasm, your sphincter usually contracts very rapidly and may have released this semen, had it been left in you. It might also be a clear/whitish like fluid that is sometimes excreted (can truly look like semen, but it's not). If this is the case, just as above, your sphincter most likely contracted, allowing this excretion of sorts to be released. I can almost gaurentee you that you weren't ejaulating from your rectum.