[TeresaM]

Since this is a new behavior, I would think there has to be a reason. Do you attach the leash to the collar or to a harness? If to the collar, please stop and use a harness. These little dogs don't have enough body & muscle around their necks to protect their trachea's.

If you are using a harness, and just have the collar on for id tags, it's possible he got it caught on something and has made his neck sore. Can you gently message his neck and see if one area is more tender.

What area was the shots in? Could he be sore from them.

Bottom line, if he was my pup, I'd keep the collar off until I knew why the sudden change in behavior. Please keep us posted.