The same thing is happening with my fish.
I bought him from a Walmart in April of this year as a b-day present for my grandma, but she couldn't handle it. She didn't like changing the water because she was "scared" she would hurt him. So I adopted him.
This is my first fish.
He has a one gallon bowl. For a water change I would boil tap water and let it sit for a day. The first three months I was kind of lazy with it and would go a week before changing it. Then during the hottest two day stretch of the summer he got fungus. He wasn't eating and he was in terrible shape. So i did some research (I wasn't educated about bettas at all) and learned that there is fungus remover. I got "Fungus Clear" and it worked great. It seemed to have made him good as new.
I felt really bad about the fungus ordeal (especially waiting so long to get on the internet and then get to the store to get it) so I started changing his water and cleaning everything every 2 -4 days. I also started giving him more attention rather than just when I fed him.
I've been feeding him the same food and he's always liked it (the piink flakes with those little shrimp things)
So anyway, he seemed fine until about 5 days ago. There doesn't appear to be anything visably wrong with him, like fungus, or fin rot or parasites or anything like that. What is wrong though is he has not eaten at all. I really don't think it is that he is being picky. He always liked it before. Also, he still gets excited when I open up the container. He comes up and eats a bite, but then spits it back out 5-8 seconds later. Then he gets this look like he realizes there is something wrong with him and sinks down to his rocks and looks depressed for a while. If I wait a few hours or especially overnight I think he forgets about whatever his problem is because he'll be happy to see food and he takes a bite but always with the same frustrating outcome.
I get free heat so It's pretty warm in here (about 75). And besides back in April and May it was more like 66-67 and he was fine so I doubt he's too cold.
So yesterday I researched for a while and went and bought water conditioner, aquarium salt and frozen bloodworms. I changed the water, put the recommended amounts in and offered him a bloodworm which he didn't even try to eat. Anyway, the salt hasn't seemed to help.
He has pooped three or so times that I have noticed since this happened so I don't think it's constipation.
Tonight or tomorrow morning I'm planning on getting betta revive and some medicated food (maybe it's an internal parasite/problem) although if e doesn't keep it down it will be pointless
Anyway, I've really grown to like him and I hope he comes around. I never realized fish can have so much personality.
I'm not holding my breath though. If/when he gets really bad I think I'm prepared to euthanize Mufasa by brain destruction.
I don't think I would ever buy a fish again from a store because it just leads to another one living in there horrible conditions.
When Mufasa passes (hopefully not soon) I think I will only get a fish if it is from someone that doesn't want it or can't take care of it...