180-degree behavioural change in dog - depression?

I have a 6-yr-old jack russsell who is usually quite active and happy to go for walks. 6wks away, his behaviour changed overnight - his tail became constantly between his legs, refuses to go for walks, and always hides under the bed. We have to drag him out for his walks, and even so, he can just sit and refuse to go anywhere, even when he is outside. When someone comes home, he will still come out and greet us, then hide back under the bed. He has been eating, albeit not as much as before. Certain times, he will also scream, but we have not been able to pinpoint the problem. We brought him to the vet who did a blood test on him, but didn't find much abnormalities. The vet also examined him for possible pain in the joints and bones, but nothing came back. If anyone has had similar experiences, pls help. It's heart-breaking to see a normally active and happy dog become so sad and lethargic