There isn't sufficient information to suggest a specific diagnosis as the lesion would have to be seen. There's a possibility of it being tinea cruris. I would advise you to meet a dermatologist so that you can be examined and undergo the necessary investigations to find the underlying cause and have it treated accordingly. These steps may help: - Reduce your exposure to the sun. Exposure to sun may trigger or worsen an episode and a tan makes the rash more visible. - Do not wear tight clothing. - Change undergarments regularly - Wear breathable fabrics such as cotton to decrease the sweating. - Keep areas where there is sweat accumulation dry regularly and apply powder in those areas to reduce the moisture.