Its called greening or whiting. Ive had it happen to me twice, it happened to me the 1st time I smoked weed and one night when I smoked a 1/4. It happens when the body isnt ready for the amount of thc thats about to enter it. Good weed can have this effect or a large amount, also adding "spin" (cigarette) can cause this as it gives intense headspins to those who dont usually smoke.