Hi,

Kindly describe little more in detail, is it itchy or is there any flaking of skin from it, also  duration.

Considering the description provided it can be simple fungal infection which is very common in folds , thighs, lower back where sweat retention is high.

Try wearing loose cotton clothes, be in an areated environment.

See a Dermatologist at the earliest 

It may be post inflammatory hyperpigmentation. it needs to be diagnosed properly after examination and dermoscopy. Dermatologist are best person this. Visit one.