Diarrhea, light headed, head ache & hot flashes. could i b dehydrated. or vitamin c interaction from 1 tablet last night? didn't eat much today...

Certainly could be: Dehydration is always a concern with diarrhea and would be the most likely explanation for developing light-headedness after/with a period of diarrhea. The headache and heat intolerance may be related as well but are less likely to improve simply with hydration. The vitamin C is not a likely cause here. Drinking plenty of fluids, water, sports drinks, thin soup/broth, will be important.