Welcome to healthcare magic.
The most common causes of an allergy
around the waist are nickel in the belt buckle and elastic from your underwear. Nickel from the buckle can permeate through the clothes and elicit an allergy in susceptible individuals.
I suggest you get a test called as a patch test
done to see if your are allergic to above components.
You have to avoid the offending allergen to find a permanent solution to your rash
Hope this helps.