Well, that's no good. Sounds uncomfy. How long has this been going on? There is a rash that specifically gets our folds of skin. Armpits, under breasts in women, and butt cracks . . . all can be a victim. It's called intertrigo. https://www.webmd.com/skin-problems-and-treatments/guide/intertrigo-symptoms-causes-treatment-risk_factors_#1 Moisture, heat, lack of circulation and friction ALL lend themselves to this developing. You should get checked out by your doctor or urgent care if you don't have one. This isn't serious but can be treated with a topical such as a steroid topical or antifungal, whichever your doctor thinks is best for you.