12yr. old pom with swelling around the front half of body and yelps after waking up

My 12yr old male pom has starting having these symptoms, will hold his head down to the ground as if he cant raise it, his back appears to arch and will also lift his left paw and he will yelp when he wakes up from a nap. It also appears the front half of his body, shoulders, neck, face and front legs are swollen. After he walks around for a few mins. the pain seems to go away and he can lift his head and his back straightens out and will play around as if nothing is wrong, but the swelling is still there. We have felt around his whole body and don’t feel anything unusual other then the swelling. He doesn’t yelp when we touch him. He still eats and drinks like normal. Other then this issue, he is a healthy, happy, full of energy dog. What could be causing this issue?

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