There are several reasons why your dog may be scooting. Often they just have an itch and this is an easy way to scratch it. In many cases it has to do with their anal glands being full or irritated and they are trying to get them to express. If the glands are infected or impacted they may not be able to empty them normally, when they have a bowel movement the glands express during that time. Also it is possible that she has worms of some type which can cause irritation and make them scoot. It would be best to have her seen by your vet to determine what the cause is, they should do a stool check to make sure that it is negative and they can express the glands at that time also. If everything is normal then you may want to learn how to express the glands yourself, it is very very smelly and takes a while to get rid of the odor so you may just want to have your vet do it as needed.
Here is a link that explains about anal glands and how they work. I hope it helps.
http://www.marvistavet.com/html/anal_sacs.html


