I know exactly what kind of heater you're talking about... I hate those things. It's one of those heaters that just constantly adds heat to the water. Good that you have it off right now, because those heaters are dangerous and can't be left unattended. I don't understand why pet stores even sell them. I got one of those and returned it the next day, ended up getting another one for only $1 more that has a temperature sensor in it, and automatically turns itself on and off. It does a much better job of keeping the water at a consistent temperature, and there's no fear of it cooking my fish.
3.5 gallons is just fine for a single betta. As long as there's no other animals in the tank.
Do you have a water test kit that can test for ammonia and nitrites and nitrates? If not I highly suggest you get one. I know you said your "readings are perfect", but we need the actual numbers from those readings to see if anything jumps out as alarming to us. Pet stores will test your water for you, generally for free, if you don't have a test kit yourself. If you have the pet store test your water, make sure you write down the actual numbers from the test. Unfortunately, pet store employees (students working for minimum wage) don't really understand what the numbers mean, and far too often I've seen them tell someone their water is "fine" or "perfect" when in fact the water was unsafe for fish.