My butt itches really bad?

It's probably a fungal infection/overgrowth. You probably need a prescription for the cream and maybe an oral anti-fungal as well; however, before you make that last step, try this:

Get some "Selsun Blue" shampoo, shake it really well (like for about 3 minutes, go by a clock, not just "It feels like 3 minutes"), Then pour-out a small amount on your hand, and using your fingertips, smear it around the itchy area.

Let it dry for about 15 minutes, then shower it off. Do this twice a day (morning and evening) for about 2 weeks.

If it's not *starting* to get better after 3-or-4 days, it's probably not going to work at all, so you can quit and go to the doc.

If it stings when you first apply it, that means that you've scratched too hard and have made a little raw area. If it doesn't sting too much, just tough-it-out. If it stings too bad, just go straight to the doc (well, finish your shower first, k?)

if its just the 'crack' its possibly a fungal infection. if its closer to your anus you may have pin worm which you will need to take a couple of tablets to treat, available over the counter at any pharmacy.