If it is raised it could be what my daughter had, an allergic reaction to dairy. She had a similar rash on
her right arm and it took a while for a diagnosis...it was a ctually my watchful eye that helped find the diagnosis. Her doc kept saying allergies to dairy don't produce rashes like this! Anyway, if the rash is flat, and appears to be below the skin, it could be "petechiae" and that could be caused by systemic illnesses. Your doc could do bloodwork to rule to rule those out.