Did you run out of conditioner or did you never use it?
Not only will it remove cholorine but it also removes heavy metals that are in the water. Without it, what's in the untreated water can be deadly to the fish. I learned that lesson the hard way - petco told me i did not need conditioner and my fish all died within 24 hours
How often do you do water changes?
Heaters - unless you live in someplace like Thailand, heaters are also mandatory since they are tropical fish. They can live in colder water but - they won't be happy with it. In warmer water, they are alot more colorful and active. Plus long term exposure to cooler temps compromises their immune system.
If you have to - you can hold off onthe heater for a little bit but water conditioner is a must for right now. There are plenty to choose from. Stress coat and prime are pretty popular and it does not need to be betta specific.