How do I figure out what I am allergic to that is causing a horrible itchy rash?

Get checked out! If the rash consists of raised lesions called urticaria (hives), it could mean you're allergic to a medicine, a food, a virus, or something else. If it's a flat (macular like measles) rash, it could be from a medicine or virus, but probably not a food. If you take daily medicines, your prescriber can help you to know which might be the most likely culprit.