What would cause a girl who is not a virgin to bleed during intercourse?
Her hymen could be still partially intact, and just be tearing more each time. Lubrication could be another big issue, is she's not wet enough the friction can cause mild internal damage.
Adding to Tiffanys answer,
It could also be because you are doing it to hard and to fast.
Get her more lubercated
Try different postiions
Its cause your going to rough or she might be starting or ending her period...take it easy on her!!!:)
thanx ill buy some k-y and try and be more gentle