You could be reacting to something external (like a weird soap or skin cream or something).
But if not, skin issues are almost always toxin or liver related. When your body can't do anything else with a toxin it gives up and pushes it out through your skin. So something you're eating, drinking or smoking.
The other possibility is some kind of auto-immune reaction like Celiacs or Chrohn's or Lupus or (fill in the blank, medical science doesn't really understand auto-immune, there are endless varieties). If so it's still probably getting triggered by something you're consuming. The hard part is figuring out what :-)