1. Heat/sweat rash or friction irritation: often after sweating, humid weather, tight clothing, exercise, or prolonged sitting.
2. Mild folliculitis: inflammation of hair follicles, sometimes triggered by sweat, friction, shaving, oily products, or bacteria/yeast. It may itch or become tender, and can develop tiny white-headed bumps.
3. Contact irritation/allergy: a new soap, detergent, body lotion, fragrance, fabric, or topical product can cause a similar rash. Avoiding the trigger is the main treatment for contact dermatitis, according to the AAD guidance.
4. Less likely, keratosis pilaris: persistent rough follicular bumps, usually on upper arms/thighs and often not suddenly appearing.

What to do now

• Keep the area cool and dry. Shower after sweating and change out of sweaty clothing promptly.
• Wear loose, breathable cotton and avoid rubbing, scrubbing, loofahs, or shaving over the rash.
• Stop any new fragranced products, heavy oils, or recently changed detergent for 1-2 weeks.
• Use a mild, fragrance-free cleanser and moisturizer.
• If the bumps seem follicular or acne-like, you can try a benzoyl peroxide 2.5%-5% body wash once daily for up to 1-2 weeks. Rinse well, reduce to every other day if irritated, and know that it can bleach fabrics. Do not use it on broken skin.
• For itch, cool compresses can help. A non-drowsy antihistamine such as cetirizine may help some adults, if safe for you to take. Avoid applying strong steroid or antibiotic creams without an in-person assessment.

Seek urgent medical care if

• You have fever, feel unwell, or the rash is rapidly spreading
• There is significant pain, warmth, swelling, pus, boils, or red streaking
• The spots are purple/red and do not fade when pressed
• You have facial swelling, breathing difficulty, or widespread hives
• It does not improve within 7-14 days, keeps recurring, or began after a new medication.