My cat is acting strangely, please help.

My Maine Coon which is about 14 years old has suddenly been acting moody and hiding lately. He usually is up and about in the morning in our kitchen, then at night or when he wants to be alone he will go down to the basement, but he is in the basement all day lately. This is highly unusual for him and I'm wondering what the cause could be. I took him to the vet yesterday and he said the only thing out of ordinary was his bladder being quite large and mentioned that it could be something to do with that and to monitor his litter to see if he is urinating properly. Last night I checked and he pooped and urinated in the litter so I think that can be ruled out? Anyways, right when we got home from the vet he went down stairs and back up quickly to the kitchen like everything was normal. I had petted him a bit on his head and he was purring like usual, but then my brother petted him (he has a good relationship with him as well) near the stomach he growled and hissed very angrily then ran to the top of our chair and laid down. When he was at the vet and the vet was grabbing the same area he didn't respond violently. He proceeded to lay on the chair for the remainder of the day without moving once. We tried feeding him Fancy Feast, but he only took a few bites. At one point when he was eating again my sister walked down the stairs right next to the chair and he growled very loudly and angrily again. Later he went down to the basement and that's when I checked his litter and left him alone for the night. Today he hasn't been up and about as well and I went down to the basement to look for him and he was hiding in a place I had seen him before maybe once. He gave me a faint meow with hardly any power behind it. This all happened very suddenly, he has been active and friendly up until two days ago when he started growling and hiding. I was wondering what my concerns should be based on the circumstances. Thank you for your help.