Sounds to me like you dumped a totally unecessary white butt ton of food in the bowl/tank/wherever he lives, poluted the christ out of the water, and your fishy probably got ammonia burn and now his gills don't function properly. My guess is the food sat there and rotted, and since he's probably in a small container it got pretty befouled over 3 days. You'd have been better off not feeding him at all, a betta can eat a few flakes or pellets a couple times a week and be totally healthy.
Not to flame but stuff like this seriously annoys me. I mean I know it's just a $5 betta but if you're going to get a pet learn how to take care of it properly even if it IS just a $5 betta.
The best you can do now is just keep the water clean, do not under any circumstances put any more food in the tank, and see what happens.
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