Thanks for providing more information.
Well as you describe the area is moist and you have history of prolonged hours of sitting so possible causes may include candida yeast called candidal intertrigo,Dermatitis,pinworm infection(if you have associated itching around anus).
So you need to be seen by your nearby dermatologist/Proctologist for a clinical examination.
In the meantime you can try some self care measures like
Try OTC 1% Cortizone cream + 2% Miconazole cream: mix and apply twice a day over the affected area for 14 days.
Take Tab Claritin 10 mg twice a day for 7 days and then once a day for 7 days more
Keep the area as clean and dry as possible.
Zeasorb-AF powder (Clotrimazole) for dusting: i.e. apply over the affected area before wearing your undergarments. Continue for 4 weeks.
Avoid prolonged hours of sitting and take short breaks.
As you have a recurrence,I would recommend getting your blood sugar levels checked as Diabetes is a common cause of recurrent intertrigo.
As this area is prone for the fungal infections so further confirmation can be done by a culture / swab taken from the area.
And specific medication can be prescribed as per underlying causative agent.
Hydrate yourself with plenty of fluids.
Wear cotton clothes.
I hope this helps.
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