Hello,
It can be due to jock itch or tinea cruris or sweat dermatitis. Keep the area dry, wear loose fitting clothes, and change undergarments at least twice a day. Sweat dermatitis usually responds to such treatment along with oral antihistamines (OTC ones). For fungal infection many treatment options are there like Clotrimazole 1%, Miconazole nitrate 2%, Tolnaftate 1% and Butenafine hydrochloride 1%. Many OTC products containing these salts are available, your pharmacist can help. Try a combination of two and apply until the itch disappears.
Continue application for 2 weeks after the itch disappears. The cause for persistence is that the treatment is often left in between. After the itch and dryness goes, you have to continue applying for at least another 2 weeks for the infection to be completely cured from the lower most layer of the skin. Test for diabetes as often this is the cause of fungal infection. If all these measures do not help, then you will need to consult a dermatologist or skin specialist to find the cause.
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.
This sounds similar to an issue that I am having. I've tried antibacterial creams, antifungal creams, and even Zovirax (for herpes virus) and nothing seems to work. I would love to find some sort of a solution as well.