I'm not a vet or anything but this reminds me of my cat a few years ago when he had a urinary tract infection and a blockage. One day he didn't want to eat, hid under the bed, was straining while trying to use the bathroom and would meow a little like it hurt. When I finally got ahold of him I could feel his bladder like a water balloon. He had to have a catheter and medicines/fluids for a week or two to help dissolve the buildup.We are really worried about Finn. Today, it was getting late and we always get him in by about 5 p.m. since he can't see well. He was evading capture and acting strangely. He was running around through the leaves, up and down the bank with us trying to surround him and risk getting hurt on the slopes. He'd stop to pee/poop, then apparently didn't because he didn't cover it up as usual, then he'd run away and meow the entire time. He even scooted under a storage barn, which is very low to the ground and I had to use the hose to encourage him out again. He is just acting oddly. I dont think he feels good at all. He will be here 11 years the first week of November, and because he was about a year and a half old, per vet, when he got here, he's about 12 1/2 now. We may have to take him to the vet. He's been eating fine. We have some of that hairball stuff you squeeze into his mouth so we'll at least do that; can't hurt, might help.
I hope he's okay, but at least we finally got him to go back in the house and he's curled up in his box (low sides, small quilt for comfort) under the side table in the living room.
I'm not saying this is what's going on with your cat, just relating my kinda similar experience. I'd probably be very vigilant about when/how often/how much he's going to the bathroom and maybe feel around on him a little to try and see if his bladder is any bigger than normal. A blockage is serious so if you think he has that or a UTI he'd need vet care.
Again, I'm not a vet and I hope that's not what is going on. Just something to think about. Either way I hope Finn gets to feeling better soon!