Unprotected sex, bright red blood before period?

Hey, sorry for the tmi, but I had sex (nothing particularly rough, so ruling out any tears) with my long term partner around two weeks ago, and took my pill half an hour outside the three hour timeslot. Today I suddenly started bleeding heavily for no reason, bright red blood with very small clots, unlike period blood and too early for my period, accompanied by crampy lower back pain. I was just wondering what the cause of this is, because its worrying me a little :)
Thank you :)