What does it mean when you bleed during sex when you're not a virgin?

There are many: Possible causes for coital ; post coital bleeding. It is not normal and warrants medical evaluation.

Blood=tissue injury: You have provided little to go on. Your data doesn't list your gender or what type of sex you engage in.Yet the basic answer is bleeding represents tissue injury which should not be part of regular sexual activities. You need to have this evaluated by a physician.