Medically reviewed by Casey Gallagher, MD Fact checked by Khara Scheppmann Armpit rashes can be caused by allergic reactions, ...

That summer was the last time I felt normal in my skin — then the rashes appeared. It was a few months into my freshman year of high school, I noticed small, red rings forming on my right arm.

However, it can also be quite large, extending over most of the arm. Reports of rash size have varied from study to study.

The morning after the itchiness started, her upper arm was red ... told CBS8 she had a rash suddenly show up eight days after she got one of her Moderna shots. She described it as a "a big ...