If it's been going on this long through all these drugs, the source is probably something you are continuing to come in contact with. It might even be something you've used in the past and just recently become sensitive to. Which means you need to do some detective work.
What kind of soap do you shower with? Do you ever put any perfume, lotion, powder or any other product on that area of your body?
Since it's an area normally covered by clothing, consider laundry habits. I only use detergents that call themselves "perfume and dye free." I never use fabric softener, I use bleach only for spot-treating stains, and I dry my clothes in the dryer so they don't pick up pollen.
Or, it could possibly be the clothes themselves, though it's usually new clothes that haven't been washed yet.
I've even heard of people having allergic reactions to toilet seats that have natural wood resin!
