Bleeding during sex

Hey y'all, I experienced something that has never happened to me until last night. My boyfriend and I had just finished having sex and he asked me if I was on my period? I'm not, so I told him no and he's like well you're bleeding. We went to go take a shower and sure enough I had been bleeding during sex. Lube was also used so I wasn't dry. I've never had that happen before and I'm not on my period so I'm confused as to how it happened, if it's happened to you gals before and what conclusions you came to.