First of all, am I the only one who thinks it's funny that someone whose name is "Scooter" is talking about a dog with itchy butt?? LOL.
Anyway, sometimes they scratch when they have worms, watch her poop especially for tapeworm, it looks like little grains of rice.
More commonly, it means that her anal glands are impacted. You'll have to take her to a vet or groomer, and they express them. It's not a fun job, and can really hurt the dog if you do it wrong, so unless you're experienced, you really need a professional to do it for you.
Sometimes it's just allergies or a yeast-type infection. My dog used to "scoot" a lot - no worms, good anal glands - and I started feeding her some plain yogurt - helped the problem tremendously.