Hi, I can understand your concern for the non-itchy and painless rash on waist. After reading the query, in my opinion it appears to allergic dermatitis. I suggest you to apply medium potency steroid cream application because are best effective in these condition. I would have opted for either mometasone or Fluticasone or cortisone cream. Do not pinch, pop or scratch or een touch the area unnecessarily as it can cause aggravation of the rash. Wear cotton clothes and cotton inner garments which will give a soothing feeling to the skin. Stay away from extreme of environment such heat and humidity. Take care.