Ok I have often had this in the mornings.
I was working in a greenhouse two summers and this often happened then and when I went to school.
It has only happened when I have woken up, have to go up fast, eat something and go out. Not a long time since lying in bed and then rushing out.
I don't know if it is related but when I stand up sometimes I start getting light headed and see black. I have to lie down for a moment if it is bad but sometimes enough to just look down.
I'm going to write down what happens:
See black suddenly
feel like puking
Sweating (like on the forehead)
Pounding heart beat
White and cold face
Cold brain feeling
Related to this or anything else I feel like I'm loosing my memory.