I typically start with plain coconut oil (yep, the same kind that I cook with!) and see if that helps to reduce inflammation and itching. If not, I move up to the A&D ointment (you want the gel-type cream, not the pasty one!)... rarely, I've had to use hemorrhoid cream that was medicated, but the coconut oil usually clears up itching for me. Whatever you use, you're just looking to put a little bit on a piece of toilet paper (after you're nice and clean, of course) and wipe it on to cover the affected area.
@Bueller My guess is a hemorrhoid or anal fissue became irritated and the extra cleaning has irritated it more. My go to product for anything below the belt is A&D Ointment/Diaper Rash Cream. (Generic is fine for most things but I recommend brand name for this.) Great stuff and gentle enough for baby bottoms!