Or possibly, your pup is simply bored. When does thebehaviour manifest itself?
Or possibly, she's is signalling she's anxious; our newly adopted rescue spent a lot of time in a kennel. she only pees when her feet are wet....she gets on her paws when she needs to go pee.
We are working on interrupting the behaviour by making sure she's engaged...special toys she only gets when we are playing together, chewing on her bully stick, chewing on her kong, etc. Maybe make sure she has other, delicious things to chew. Some dogs just need to chew. Distract, distract, distract..and praise her when she is chewing something appropriate.
Other posters have offered some great things to research, as well. Try everything (one thing at a time, though!!). Check out all the health related things first, of course.
Regarding allergies. My friend has a Jack Russel she purched when the dog was two. She had terrible allergies. Awful. My friend put her on a good kibble and turkey neck diet. She sprinkled GRAPE SEED (powder in a capsule, break open over food) and her dogs skin issues cleared up dramatically. When the dog occasionally has a breakout, it's back to the grape seed powder and within a couple of days, the dog is no longer frantically gnawing, scratching, itching behaviours.