Ok girlfriend is mad at me ? Supposably I forgot her birthday?

If you have money go buy her some flowers, a nice bday card, and a small cupcake with a candle, or a teddy bear instead of a cupcake. :) Tell her you are extremely sorry and you did not mean to forget her birthday. Ask her if she can forgive you. If she says yes then give her a kiss if not try to convince her that you are truely sorry and will make up for it. Hope this helps and best of luck!