What Causes Dizziness, Nausea And Bloodless Feelings While Standing?

once i had a dizzy feeling while i was standing for two consecutive hours in physiology practical class >>then i felt like some one has flushed the blood out of me >> and had nauseatic feeling and then when i cudnt stand i managed to sit and i was ok after few minutes >>doctor was it physiological or is there any pathology inside me>>>what do u say doctor