This is a very very large penis that is often extremely uncomfortable/painful for some women.
I would recommend taking it slow if it is painful for her. Maybe you can buy her various sizes of dildos and insert them in her until she gets used to the size, slowly working your way up.
The bleeding is most likely due to either irritating her cervix (which would be by pushing in too deep which is very plausible due to the length), or because she was about to start her period anyways.
To prevent the pain/bleeding this way, I would just not push in as deep. Have sex entering her very slowly. Use a lot of lube; it will help out a lot. Put in 5 or 6 inches and see if she is feeling any pain and slowly keep entering more until she says she feels pain. Then just try having sex only using that much of your penis.