My vagina itches when i wear thongs. what could be causing this?

Depends: What is the material of the underwear? Cotton breaths better than synthetic materials. What type of detergent do you use? Some people can develop a topical irritation from soaps. In addition, because of the movement of a thong when you walk, thongs can increase the risk of passing bacteria/yeast from the rectal area to the vagina therefore increasing infection risk.