Chest/shoulder twitching and walking slow

Hi guys and gals,

I hope you can help me. I've been trying to research this problem online and have found nothing that matches. I have a 5 year old beagle named Tony - not neutered and he's a little overweight at 37 pounds. He eats Science Diet dry food and I cook for him at night (ground beef/chicken/turkey with vegetables and wheat rice mixed together). He get's a half hour walk in the morning, an hour walk in the afternoon and he plays in the dog park for about half an hour at night.

He's always been very active and healthy but this past week, I've noticed him acting a little weird and it's really starting to worry me. Right after he poops outside, he starts walking VERY slow. He even stops and refuses to keep walking. He's never done this. I also notice his heart/chest/shoulder start to twitch a lot. He also has a very tired/depressed look on his face. He's usually a very hyper and happy dog full of energy. I'm not sure what is wrong with him. Any ideas?