A rash is irritation showing on the surface, read as emotions or annoyance that are hard to contain. It may also mirror a sense that something got under your skin.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.