I've been sitting here wracking my brain for about ten minutes just trying to think of what could be affecting that area of your body that wouldn't affect the other areas. Perhaps it's your underwear, pants, or some body product you're applying? Another thing to keep in mind which is kind of annoying is that different body parts CAN react differently. For example, my right earlobe has always been much more reactionary to nickel than the left earlobe.
People can react through their skin for more internal reasons - Celiac and stuff like that. But patch testing is a great place to start. It changed my world! Avoiding your allergens can be a pain, but I believe that it's a much better option than steroids. It requires a lot of diligence, but it's worth it! I actually did a little bit of research on propyl gallate a while ago because I saw it in brats and wondered what it actually was. Check that out here.
Hang in there and try to stay positive! You can do this!