I get this rash every time i’m on a diet

It starts 3 weeks after i start a diet and its VERY ITCHY only only on my chest between breats like in pic . I had it 10 years ago on my first diet. It lasts around 2 weeks then it’s gone for the rest of the diet. No doctor could tell what it is help any ideas

It starts 3 weeks after i start a diet and its VERY ITCHY only only on my chest between breats like in pic . I had it 10 years ago on my first diet. It lasts around 2 weeks then it’s gone for the rest of the diet. No doctor could tell what it is help any ideas