welcome to healthcare magic forum
thank you for your query
it may be intertrigo
or fungal infection
chance of intertrigo is high at site you described
you need to keep the skin clean and dry
for short time you can apply steroid
and antifungal cream .
better to consult dermatologist
for proper treatment
do not scratch or popped the lesion
keep the weight under control
wear loose and cotton cloths to prevent friction .
we should find out the underlying medical problem also .
i hope i answered your query