I would first try to determine if the problem is air under the skin, or something in the crop. If she does it only when eating, it may be crop-related. If it is done all the time, it could be crop or neurological. But I have never seen this before. A vet visit might help if you cannot figure it out. With crop problems, massaging the crop, feeding fluids and a little plain yogurt may help. TwoCrows has a good article about "treatment and prevention of impacted and sour crop problems," which you can do a search for here on BYC.