I've Been On The Depo Shot For Three Years Now,

I've been on the depo shot for three years now, I started having sex last year with my boyfriend. We've had unprotected sex and protected sex. But just recently, we've had unprotected sex. Two nights ago, he came inside me and after that I started bleeding. The bleeding continued and then stopped. And now we just had unprotected sex and again I started bleeding during sex. What is the cause of this? I've never expiereneced this.