hi, i help out at a dog groomers and also have a dog prone to scratching myself,
i couldn't help you with the cause but the main thing to remember is that scratching/licking behaviour is very addictive, so even when the cause is long gone the dog may still scratch,
so, try not to give attention when she scratches (like..oh poor doggie, is it sore there there) but instead without making a fuss move her muzzle from the area gently and try to get her to relax without making a fuss (not easy and can become quite infuriating!!) you could try a lampshade collar from the vets so she cant get to the area or if its possible bandage the area, maybe put some soothing aloe on first, (mine likes to scratch his ears so its fun trying to stop that!!!)
good luck with finding the cause and hopefully you will all get some rest when the itching subsides