What Can Cause Fainting During Menstrual Cycle?

I keep fainting during the start of my menstrual cycle. This normally happens during the early hours of the morning. I begin to feel weak, clammy, nausea and excessive sweating. I have spoken to my gp and he has suggested that I go and take a urine sample to him once my cycle has finished. What could be causing this. This is also accompanied by cramps in by back & stomach.