welcome to hcm.
in waist commonly due to fungal infection
chronic irritation may cause rash also.
i think food is not related to it.
try to avoid rubbing that area.
put the band of underwear just below or above that portion.
you can take antihistaminic tablets for itching.
can apply candid
cream twice a day for 10 days.
probably these will help.
if not you should consult a dermatologist