Hi! I Have An Itchy, Raised Rash Only In My

Hi! I have an itchy, raised rash only in my inner, upper arms. The rash does not show up at all where I apply my deodorant. I have not started using any new soaps, deodorants, detergents etc. It is slowly spreading down my side and arms. I am currently on Cephalexin (day 10) as a post-root canal precaution. This rash showed up 5 days ago. I am otherwise is great health, healthy weight, and have no other symptoms.