Welcome to HCM
We understand your concerns
Rashes are due to skin irritation as you already know. It may not have happened only because of cheap brand of bikini briefs. It may be associated with dry skin
, reaction beetween sweat and washing soap, chemicals used in water, external allergants etc.
The rash will disapear its own if you keep the area moist and clean. Wash the area with mild moisturising soap and warm water. Use moisturising crream. If need be consult a dermatologist
Wish you speedy recovery.