HI ,thanks for using healthcare magic
Normally drug reactions would present with a generalised rash, it would not be limited to one aspect of the body.
The swelling is likely part of the inflammatory reaction of the rash, which is likely to be a diaper rash
. A topical cream combining an anti fungal with a mild steroid
(to reduce the inflammation) would be helpful.
In addition a barrier cream would be needed to apply to the area.
I hope this helps