Bleeding after sex...

Has anyone had this happen before? My partner and I have been having sex for a couple of months and it has never happened before now. We had a lot of sex last night (3 times, rough at times) but nothing out of the ordinary. Nothing was hurting or anything. He has very large penis but it wasn't the first time with him so I'm stumped. It was dark, thin blood and not like a period at all. Should I worry (because obvisously googling it gave me cancer...)?