What Causes Lower Abdominal Pain After Unprotected Intercourse?

I am almost 19 years of age and have been having unprotected sex with my boyfriend only a few times here and there (about 3 times recently). I have been taking the day after pill each time. I have started to get pains in my lower stomach and took a few pregnancy tests yesterday to make sure I'm not pregnant. the tests came back negative. Today when i used the bathroom i noticed a thick, black, tar-like blood on my underwear and am not sure what that may mean. Ive looked up a few different things but I can't figure this out.
-Allie