What was the temperature of the new water? I once almost boiled one of my bettas to death when I noticed him acting funny I realized the water (out of the tap) was too hot and was killing him.
Anyway, my cousins put a betta in with goldfish, I went over to visit them and saw it on the bottom of the tank, I thought it was a salamander or something. All of it's fins were chewed off and it was sitting on the bottom, I have no idea how it was getting any food.