What Causes Red Rashes On Arm And Thumb Pain?

I have developed a different size red rash that is on my left arm only. The large part of my thumb aches and various parts of my arm up to the top of my shoulder.The pain is considerable. Please help. This began Friday night with pain first at my wrist then began to break out Saturday. I am 81 female withgood health I do not have high blood pressure or cholesteral. I weigh 120. I am 5'4. My main problem in the past has been terrible insomnia...I have been helped with Ambien 10mg. I have been out for several days.