Actually it is more my arm than my underarm. Anytime I do a long cardio workout, I get a burn spot under my arm and it stings really bad. Is it because my arms are fat or what? And how can I prevent it? I was doing Rhythmic Step today and was proud of myself as I was almost finished with the entire workout, but I had to quit during the middle of the challenge because I couldn't stand the stinging any longer.