hello and welcome to e.health forum. this is a problem that many of us women suffer from and nobody really knows the cause. sometimes the if underwear is not cotton it can cause us yeast infections and we get thrush. i blamed tampons on mine.
if i were you i would if its only your thongs change style of underwear but if it persists i would get hecked for any vaginal infections as bacteria is another option to look at.
there are plenty of posts in the forum about same thing as i posted one myself.
hope this has helped you in some way.