My dog is nearly 4 and she has regularly had this problem. We smell that smell and she licks her bottom. Oh it's awful. She never scoots across the floor - just suddenly aware of fishy smell and then licks.
Having googled the suggestions are her anal glands need to be emptied by the vet - hmmm we'd be going once a week!
Or give her quality dog food - she already does have that. Is there anything else I can do?