What Causes Dark Patches In Underarms And Groin Area?

I have dark underams and a slightly peeling of the old skin. I also have dark patches on either side of the groin area. This has been peraiatant for couple of years but when matters became worse I consulted a dermatologist who just looked once and dismissed it as a case of fungal infection and is treating me with flucanozole twice a week for a month,ketacanozole cream and abzorb powder. Also cabthex tablets twice daily. I am not sure if the diagnosis is right and want to make sure if I have erythrasma. If the medication I am following wouldn t be adequate.please advise.