What Is The Blood In Urine After Having Sex?

I had quite rough sex with my boyfriend recently, and afterwards when i peed it stung and there was a lot of blood in my urine (definately from the urethra, not vaginal bleeding as i originally thought) these symptoms can last for a few days after, gradually lessening in severity, this has happened each time and is worrying me, is it the friction/pressure on my bladder or urethra? what can i do to prevent it happening?