It might be some sort of deep (internal)fungus issue. You probably need to address it internally,topically and externally (clothing).
Neem soap and oregano oil is really good for fungal infections. Since it's in such a sensitive area, I'd get some neem soap (online, health food store or indian market) and use a grater to shave some in a small jar and add hot water to liquify and add a few drops of strong oregano oil. Wash the area with it, let the lather stay on as long as comfortable, if it stings a lot, that's the oregano oil. After shower, you can add the oregano oil topically diluted in some carrier oil. I'd do this twice a day.
Wash all your unmentionables in hot, hot water. If everything is white, add bleach in the wash. Wash again a second time with white vinegar, it'll neutralize the bleach.
You should probably wash your pj's and sheets the same way too. If you can't use bleach, use the hot water and vinegar wash too. You can also wash with hydrogen peroxide. I've never tried that, but I've used it several times to steam clean my carpet and it doesn't "bleach" it. It's great for carpets in removing pet stains and cleaning without a soap residue. The woman who told me about hydrogen peroxide for cleaning the carpet also uses it in the wash. Get some big bottles from the drug store.
Take internally some anti-fungal, anti-parasitic herbs/kits like paragone and candidagone. You may need to do it for a few months. Add to that, neem capsules. They can be taken internally. Neem is antifungal, antiviral and antibacterial.