What Causes White Circles/patches On Skin Spreading Down The Thighs?

i have white circles / patches on my skin ( stomach ) spreading down my thighs and up to my breasts they sometimes look red and a little aggrivated, i have been given a shampoo for dandruff to try from the doctor but it hasnt done a thing, they are spreading and i am now strting to get worried please can you help? i am 27, 5f 3inches ancd weigh 10st 11pound. I have had this before approx 6 years ago and it has now come back?