I have a large red rash under left armpit. it's not raised not painful and not itchy. the entire area is red in the shape of a circle. noticed today. ?

Sounds like a: contact dermatitis to deodoorant or clothing. See a Dr. and get a steroid cream for this.

Need to see rash: Your rash being in just one armpit somewhat lessens the likelihood it's due to deodorant/antipersperants. There are a number of other conditions that can cause underarm rash. Some of them are intertrigo, Candida, other fungal infections and reaction to shaving. **The rash of Lyme disease can occur in one armpit. You can upload an image in an online consultation at Health Tap or see your physician