Lanacort - contains benzocaine to kill the itch, and cortizone to help it heal.
Assuming it's a chaffing, and not an actual infection, for that you need a doctor.
You might also try Gold Bond powder for "prickly heat rash".
if i were you i would go the doctor to make sure, just be careful he doesn't have BOTH hands on your shoulder when he performs the digital rectal exam.
Last time I had a rectal exam, I swear the lil sumbitch jammed two fingers in there. I had a wart on the bottom of my foot "removed", now I have two. I can go on for long time. I completely swore off doctors.