that sucks
hmmm, well if you've ruled out a UTI and yeast and/or B.V. infections. (and i assume you don't have any STD's)
it might be that you're not using enough lubrication during sex... did it feel dry at any time during sex? I would suggest using KY jelly, being super wet is very important, vaginal tissue can be quite fragile and become bruised etc.
If this happens again, i would suggest seeing your gyno. about it, you can never be too careful when it comes to our girly parts
sweetpea