It is difficult to know for certain what is causing these dark skin patches without seeing them. However, it is possible that you have a condition known as erythrasma. This is a chronic bacterial infection and commonly occurs in moist skin folds on the body - like the groin, arm pits, in the skin fold between the buttocks and on the inner aspect of the upper thighs where this skin may rub together.

Erythrasma is reddish brown patches on the skin that might be slightly scaly. The borders or margins of the dark areas are quite clear. Sometimes there is a slight itch. Erythrasma is caused by bacteria. Using an antibacterial soap to gently clean the areas may be enough for it to clear up. Other treatments include topical antibiotic creams or oral antibiotic tablets, usually erythromycin. Please see your doctor or other health care provider for more advice.