Bleeding during sex, could it be an std?

Me and my boyfriend lost our virginities to one another. He hasn't done anything with another girl (that includes sex AND oral sex.) I havent done anything with another guy. We've had sex 4 times, and i've bled every time. A friend told me it could be an STD causing the bleeding. Could it possibly be an std even though we've only had sex with each other?
 
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