The lab I had at one time got a cyst in her back. Took her to the vet and he cut it out but, it kept coming back. The dog digging and biting at it will eventualy cause it to break open if that is what it is. That is one thing I have noticed about some of the labs as well as other dogs, as they get older some are more prone to get cyst the some other breeds. When wehad our lab put down she had the same one that had been removed about 4 times.