From the small amount of lurking I've done here in the past hour and a half, I know you guys like to have details about the setup of the OP. I have a quart bowl (small, I know; I'm planning on upgrading) with no filtration or heating system. Room temperature is fairly stable and on the warmish side. My fish has no tank mates. I have been feeding him three betta pellets three times a day. I have done partial water changes (1/3 to 1/2 volume) or full changes every couple days (I figured it's best to err on the side of frequency because of the small bowl volume). I use no additives, but filter my water with a Brita filter and let stand in water bottles at least overnight. I have not tested my water, but the sudden nature of this problem suggests it isn't an issue with the water quality.
I bought almost a week ago my first pet, a peppy little betta I named Aroon. He had been very healthy and happy, blowing bubble nests every night, eating his food enthusiastically, swimming energetically, and even learning to recognize me. This evening I decided it was time for another water change. So I scooped him out of his bowl, rinsed the gravel, poured in some tap water I had previously cleaned with a Brita filter and left to normalize to room temperature, and went to pour him back in. I noticed he wasn't moving much at the bottom of the cup I'd put him in during the cleaning and his gills were flared, something I hadn't seen him do before. I figured he was a little stressed out from being taken out of his bowl. Perhaps he'd wanted a bit more time to digest his dinner?
When I put him back in his bowl, he just rested on the bottom, unmoving, but breathing heavily. After a while he began to move his fins a little and his breathing seemed to settle down. I thought he was coming back around, but then he began having spasms of increasing violence and frequency.
There was no temperature shock from the water since it had been sitting at room temperature for days. I can't figure out what else may have caused him any trauma, since I've changed his water a couple times with the exact same method over the week I've had him. My friend did briefly stick his finger in the cup I was holding Aroon in, but I find it hard to believe he caused him any harm.
I don't know if his behavior in the cup was indicative of the start of the problem, or if it was simply stress that was followed by some actual injury in the bowl.
He's now thrashing about every second or so at the bottom of his bowl. I fear that by the time any of you responds to this, your comments will more likely be speculative autopsy than treatment advice. I am planning to get another fish and if anything I'd like to learn from this if there is anything to be learned.
*Edit* Unfortunately, it appears that Aroon is now quite dead. I would still appreciate any comments you may have so that this can be a learning experience.