Hi,
This could be an irritation, infection or intertrigo. In particular, intertrigo is an irritation of touching skin surfaces in body fold regions (armpits, under the breasts, belly, buttocks, groin, and sometimes between fingers or toes). It can be worsened by any conditions causing increased heat, wetness, and friction. It may also be complicated by superficial skin infection with yeast or bacteria. Redness and breaks in the skin may be observed. The area may also ooze or be sore or itchy. A bacterial, fungal, or viral infection may develop at the site of broken skin due to the inflammation. Regularly cleanse the affected area gently with mild soap substitutes and keep the areas dry and exposed to the air. If not, absorbent cotton or fabric, as well as absorbent powders, may help. If it persists, have this evaluated further by your dermatologist for proper management. Take care and keep us posted.
I had a problem with severe itchy red bumps under my arms a few years ago, I was miserable not knowing what it was, and was finally able to associate it with my stick deodorant. As long as I use a spray deodorant I have no problems. Hope this helps you!!