What Causes Vaginal Bleeding After Having Sex?

Hi. My girlfriend and i had sex today. Immediatly after having sex her vagina started to bleed. I think there's just a tear in the cervix. She thinks she's pregnant because we had unprotected sex. But i didn't ejaculate inside of her. I didn't even have any semen leak out for the 15 seconds i was in her.

Could you please give me some reasons as to why she may be bleeding? She says the symptons are exactly like having her period.

Any help on the matter would be amazing. We are both under the age of 18 so we are freaking out here :(

please respond.