I know its obvious but you did use water conditioner to remove the chlorine I hope.
When changing water, its important to also match temperatures. You can tell within one degree if you have a temperature match by just moving your finger back and forth between the water the fish is in and the new water and feeling the temperature. A half a degree match takes a little practice but is not impossible.
Your fish needs clean warm water but it should not be any warmer than the fish is accustomed to. The match is far more important than the actual temperature up to a point. If the bettas home is not heated, it may be too cool for her comfort this time of year. That is the main reason that people use heaters in the tank. It keeps the water warm and more importantly it keeps the temperature constant. For a betta, the temperature is best between about 76 F and 82 F. Without a thermometer, you won't know what it is.