If she's pregnant, she needs to stop taking the pill. A positive pregnancy test is a .V.E.R.Y good indication that she is pregnant.
And any bleeding during pregnancy, while it could be nothing, needs to be checked out by a doctor. She could very well be miscarrying, and she needs to be checked to make sure. Some women bleed very lightly when they miscarry, and others do not bleed at all.
This is your girlfriend and possibly your unborn child we're talking about. Please play it safe and get her help. Better to get it checked out and feel silly if it's nothing than to have something go wrong and feel badly because you decided not to act.
I'm not trying to be mean. I honestly want to help you and your girlfriend. Please... If you love her, take care of her and have her looked at by a medical professional.