look into hidradenitis suppurativa. you can find many good links on google.
Hi
There can be many causes for underarm rash e.g. dermatitis (atopic, contact, allergic), heat rash, excessive sweating, folliculitis, boils, scabies, psoriasis, some infection leading to enlargement of axillary lymph nodes. Sometimes even breast cancer or hodgins disease may present with enlarged axillary lymph nodes.
You should wash the area with hypoallergic antibacterial soap followed by rinsing thoroughly, apply moisturizer, wear loose fitting clothes and avoid deodorant / cosmetics for a couple of days. Sometimes over-the-counter hydrocortisone cream provides some relief.
Since it is impossible to diagnose skin conditions without actually being able to examine the patient, I suggest that you consult a dermatologist who can determine the cause and prescribe appropriate treatment for it.
Regards.