Well, it seems (without seeing water test results) that you shocked your fish by doing too much change at once. It's always good to do daily and small changes to your betta aquarium (if you don't have filtration system) or 20-30% weekly with filtration. You never do too much stressful changes, as our fishes are very sensitive.
Let us know how it continues and please try to stabilize its environment as more sudden changes won't help.
