I should add that it normally doesn't affect my head, neck, or hands.
Hello,
One possibility for such itchy rashes which appear all of a sudden and then disappear after itching is hives or urticaria. Hives have a tendency to change size rapidly and to move around, disappearing in one place and reappearing in same or other places after some time. They are mostly in response to allergens in the environment but can also occur to hot and cold conditions.
The standard treatment of hives is oral antihistaminics and topical steroids. Oral antihistaminics relieve itching and you can get them over the counter but some of them may cause drowsiness and hence should be taken under a medical guidance. Topical steroids is available by prescription from a dermatologist.
In addition you can apply calamine lotion which will help in soothing the skin and take some vitamin C which is a general anti-allergy supplement. It helps stabilize mast cells so they are less likely to release histamine. You can also take a diet rich in vitamin C like citrus fruits, oranges, papaya, grapefruit, raspberries, blueberries, cranberries, and pineapple.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.