My dog is a 17 week old pit bull. His behavior is like a 99 year old mans with arthritis. He still is eating, has solid poop, has NOT thrown up and is still drinking lots of water. It takes him a very long time to go down a few steps. He is not playful with the other dog, won't jump onto the couch or bed and is hardly moving around. He's been having a lot of accidents in the house the last two days as well. He's very lazy and lethargic. What's wrong?