My dog is not the most behaved walker and frequently pulls and thrashes on his leash. Occasionally when he does this he will start screaming for no apparent reason and not stop for a few seconds. He does not show any other signs of pain except for these moments. He is a four year old shetland sheepdog. We do not know where to even begin looking for a problem so I was wondering if anyone else has experienced this or know of common health issues in shelties that may cause this? Any help is appreciated!