Do a 50% water change, if you forgot to rinse the plants before putting them in the water, it introduced dust particles, otherwise it stirred up the gravel, and it happens to me every time I move decorations (even without adding new ones) just don't forget to condition the water :)
Still not eating? did you try frozen bloodworms? I use freeze dried to add variety, but frozen ones are better, and bettas are more likely to eat them if they are picky eaters. Also, just a suggestion, maybe try feeding him during a different time of the day? Maybe the store had them on a very strict feeding schedule because of the employee's shifts, and you are trying to feed him during a different time? Fish can be as finicky as people.