There is a third reason for scooting.
Allergies, they can often be mistaken for impacted anal sacs, the most common of the allergens are fleas, so check and treat if necessary.
Have you changed foods recently? Perhaps your dog is allergic to an ingredient.
Impacted Anal Sacs are obviously a problem and should be taken to the vet if it occurs for the first time, many dog owners express them theirselves but this can be risky. If not properly emptied the sacs can become infected and if done incorrectly can cause rupture = pain for poochy

. (So it would cost less to have your vet do it...just in case!!)
Tapeworm is also a possibility, check faeces and the fur around the anus for white segments that look like grains of rice and treat if necessary.