Alot of conditions could be the cause- it could be a fungal infection. If that's the case, you would want to try an over the counter anti-fungal. It could be dry skin or an irritation from your laundry detergent, from your clothing being too tight and rubbing up agains you, irritating you- or a few other things.
It's always best to see a medical professional to get them to diagnosis you in person, they can ask more questions and gather more information. Hope this information helps and feel better soon!