Thanks for posting your query.
If its a medication that is causing the rash
, it would have been more widespread and not only localized to the buttocks.
The rash could be due to some other cause, however it would be difficult to opine regarding the rash without having a look at it.
From your description there could be two possibilities:-
1. Fungal infection
2. Irritant contact dermatitis
I would recommend that you apply clotrimazole
over it twice a day for a week.
If there is no improvement, then you can apply desonide
cream over it twice a day.
It would be advisable to consult your dermatologist
to come to a proper diagnosis.