I had sex last night, it was a little rough and the position we were in caused deep penetration. during intercourse, my partner pulled out and saw a significant amount of blood on and around his penis, what could cause this vaginal bleeding?

Any new or unexplained vaginal bleeding needs in-person evaluation. Vaginal bleeding triggered by sex often means cervicitis, i.e. infection or other inflammation of the cervix. STDs are one cause of cervicitis, if you're at risk (new partner, new relationship, other partners, etc). So don't delay: see a doctor soon. Good luck.

'Rough' sex can cause trauma to the vaginal area you could also have some condition which got injured. Best to get checked .