Is that a raised rash or is it flat? I couldn't tell from the picture. 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. The coming and going of the rash made me think it was like my daughter's rash. I don't know if "petechiae" comes and goes like that. We'll see what the doctor here has to say. I wish you the best of luck!
These spots are called petechiae. They can be associated with problems with blood clotting or with infection. The fact that yours apparently come and go suggests that your case isn't either a blood or an infection problem. In any case, you need to have blood tests to assess platelet function and clotting, and you should see a skin doctor as soon as you can arrange it. A skin biopsy may be needed to find out exactly what it is.
Best.
Dr. Rockoff