I Have Developed A Rash On My Right Upper Arm

I have developed a rash on my right upper arm - red areas, quite itchy. The number of spots is gradually growing. I have tried treating it with TCP neat and with Germolene, but these seem to have had no effect. My left arm is now beginning to have spots. I suffer from seborrheic dermatitis on my chest. I don't know if this is all connected?