This is a tricky one...
About eight months ago, my 7-year-old indoor only cat escaped from the house and spent a day outside. When he came in, he had a sneezing fit. The next morning we found him nearly unconscious under the bed. He ended up spending several days in the hospital with what the vets believe was a URI. When we got him home, he was still lethargic, but appeared to be getting better day by day. The only significant change we noticed was that he had stopped jumping. He used to sleep with us at night, snuggle on the couch and jump into windows - this all stoped. About a month into his recovery, he began to snap at us whenever we tried to touch his right side. This would come and go with some days being worse than others. Eventually, he started meowing ferociously and following us around the house biting at our ankles. We took him to a different vet who noticed he had pain in his right shoulder and prescribed pain killers. We figure his front leg was injured the night he escaped because the first vets had noticed that his claws on the same paw were broken - at the time, they did not notice any injury.
My cat has been tested for just about everything imaginable - thyroid, diabetes, FIV, FLV, kidney function, etc. Everything is normal. He has a great appetite and doesn't throw up or appear to have any digestive problems. He still is very lethargic, refuses to jump (except for a very occasional stay on his favorite chair), and continues to follow us around meowing. Because he's so persistent, the most noticeable issue is with his complete personality change, but the other issues are also concerning. Now, we recently noticed a change in his fur - it is very dull and he has a lot of skin flakes. He also appears to be sensitive to brushings (he used to love being brushed, now I feel like I'm hurting him when I do it). We also noticed that he feels fragile - like you have to be careful when picking him up, petting him, etc.
We took him to the vet for yet another exam and discovered that he has lost 2 pounds (in about 2 months), most of which appears to be muscle loss. He is an overweight cat - he started out at about 18 pounds 8 months ago and is now down to 13. We changed his food several months ago to a high-protein dry food for indoor cats, which initially improved his coat. The vet also noticed that he still has a slight limp on the right side and muscle pain near the front right shoulder. Although we had a lot of bloodwork run 8 months ago, our vet recommended another complete set of tests. Once again, everything appears normal.
We have no idea what to do at this point. Any thoughts on what might be wrong would be greatly appreciated.