Or maybe his fingernails are too long and/or sharp - not advisable if he's going to be playing around with his fingers in there.
Since everybody else has gone for the more complicated answers might I ask the simple? Assuming that you are possibly of only a small amount of sexual experience, are you a virgin or someone who has had only limited experiences? If so it might be that he was stretching or ripping what was left of your hymen.
The hymen can stretch to some degree so it is very possible to have had sex many times and yet still have enough hymen left to tear.
You might want to get checked out by your doctor, though it could just be that he was rough and caused you to bleed...
Cervical dysplasia, Chlamydia, Gonorrhea, Vaginitis, Cervical polyps, Trichomoniasis,
Vaginal yeast infection, Endometritis or adenomyosis, Uterine polyps, Fibroid tumors
All these can cause bleeding see your GYN