What Causes Dizziness After A Gastric Banding Surgery?

I am 22. I am 168cm tall and weigh 83kg. I have had gastric banding surgery and am on the pill Monofeme. Sometimes when I stand up, either from sitting or laying down, I'll get really dizzy and my vision will start going black but its all over in a few seconds. It doesn't happen all the time. I don't know how long this has been going on. Should I be worried about this?