Fish constantly excrete ammonia through their gills as waste, kind of like the fish version of urine. In a small closed system like this one, the ammonia builds up very fast to the point that it can burn this fish. This can cause gill problems as well as other sicknesses. The only way to remove all of this ammonia is through more frequent 100% water changes. For a tank this small, you'll probably want to change the water and clean the tank at least three times a week. You should get a liquid test kit so that you can determine with certainty that this routine is getting rid of all of the ammonia.
For now, I would take OldFishLady's advice and do some daily water changes. 88 is a bit high, so if you can find a cooler, stable place in your house, that would help also. In the future I suggest you look into getting a larger tank, at least 2.5 gallons, but ideally 5 gallons. This will make your life easier and allow the betta to have a heater and more room to exercise--please consider it.