Last Friday I had sex twice (about three hours apart), first time nothing abnormal but second time, after about 15 minutes or so, I noticed that I was bleeding. We didn't stop immediately, and at the end there was a fair amount of blood (enough to make a 10cm ish stain on the sheets). It had been two months since I had sex last, no STIs as I was checked while with with my previous partner and this was the first time I'd slept with someone new since then. Also I bled another time with a previous partner, went to see doctor who couldn't see anything obvious. Any suggestions?