Cause of itchy red rash under stomach and between legs and cure?

Fungus. Fungal infection beneath stomach roll is called intertrigo; that of the groin is tinea cruris. Fungus loves warm, dark, moist areas of body; so keeping those areas dry (change damp clothes/underwear; apply talc or antifungal spray) will help. Use antifungal spray or cream (lamasil/tinactin/cruex/desenex/etc) for at least a week after rash resolves & then as needed for prevention.