A little blood after , not my first time.?

I'm 21 years old, and my boyfriend and I have been in a monogamous relationship for a little over four years now. However, since we've been attending separate universities its been about three months since we've had . After the first time we noticed a slight bloody discharge. Since then I bleed a little each time we have . This has never happened before and I'm a little freaked out! Any suggestions on what the problem might be?