Yeah, my black lab does that. My vet says it could be allergies or referred pain (the dog is severely arthritic) -- or has just developed into a habit. Benadryl can help (and it's safe for dogs -- though they metabolize it much quicker than people do). Instead of that, though, we use Calm Coat, it's a tea-tree-based oil that helps soothe the area and tastes terrible -- a double whammy. If there's any rash or abrasion as a result of the licking, BluKot (sp?) spray will help -- just make sure you do it outside and keep him out for a few minutes till the stuff dries. They call it Blue for a reason!
BTW -- you might want to think about whether or not there's been any other kind of change. New baby? New schedule? Stuff going on in or around the house (landscaping, renovations -- that kind of thing). Sometimes stress can kick it off.