I would suggest more water changes. OFL (OldFishLady) would be able to give you more definate answers but straight away I would say more water changes. Your filter may be doing its job of keeping the water clean but ammonia (a fish by-product) can build up fairly quickly and can easily become fatal.
I borrowed this from the sticky about betta care:
A betta needs some sort of maintenance, as would any other fish. They are not "magic fish" and can take care of themselves. Here is a list to insure the best care of your betta in which you, the owner, must provide.
1) Weekly Water changes. A filter cannot take out everything in the water. Would you like to live in your own poop? This is why bettas need regular water changes.
A betta tank should never need 100% water changes unless working with medications. Too drastic of a change in water params can cause illness and death to any fish. Bettas can withstand more than the average tropical fish, and 50% changes are good for them if done frequently enough. The smaller the tank size the more frequent the water changes should be done. Anything under 2.5 gallons should have a 50% change every other day. 2.5 and more should have 50% changes at least twice/wk. If a filter is running in the tank, 50% changes once/wk are usually plenty.
2) Testing Water. You need to keep an eye out for your bettas water paramaters. Water params are nitrate, nitrite, ammonia, pH, and water hardness. A good liquid test kit will help determine what is in the water. Ammonia and nitrite should be at 0, and nitrates under 20. The pH level should be between 6-7.5. If any levels seem wrong, do a water change to get clean water in the tank.
Hope you find this helpful and here is the link to the sticky if you want to read the whole thing
Best of luck