From the provided description it sounds more like a skin condition than an infection, and as it has been a while since you havenot had sexual contact in a while, this is most probably due to either the accumulation of sweat or stagnation of urine.
I would advise you to keep the area clean and dry, either wear comfortable cotton undergarments or avoid them completely; and schedule an appointment with the dermatologist
(skin specialist) at the earliest.
Based on the provided information this coule be dermatitis
or even psoriasis
, but I would like for you not to assume any such conditions and self prescribe and visit a doctor to get treated accordingly.
Hope this helps.