I am 19 years old and have had sex with only one man multiple times since the age of 16. But we stopped, for the most part other than once every few weeks. We had intercourse once on Friday and again on Sunday, both times used protection and no lubricant. (In the past when we have had intercourse, we have not used protection) I had no pain on Friday but had intense pain on Sunday, most likely from friction on Friday. I started bleeding on Monday morning. It is not my period, because I am still approximately 14 days away. What may be the cause of bleeding?