I consistently wake up around 4 in the morning feeling sick. No!! I am not pregnant.?

I have been sterilised so I know I am Not Pregnant. My Doctor says its not a symthom of menopause either. Has anyone experienced this. It offen (5 time out of 10) is followed by a severe headache but not all the time. The nausea can last up until midday and I am never physically sick. I get this in bout of having this feeling for weeks then it stop and i feel fine again for a while. Has anyone had this and have you any tips what it could be.