This should really be posted in the disease and emergencies section here: http://www.bettafish.com/forumdisplay.php?f=99
Either way you will need to answer as many of these questions to help the diagnosis.
What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?
My first thought is cotton fungus but without a picture it's hard to tell. Even though you are doing a water change between fish the fungus spores are still in the tank. On the glass, in the gravel, on the decorations. By not fully cleaning every inch of the tank (it takes more than just a rinse in some warm water) the infection will keep killing fish.
The best treatment to try first is 1 tsp of AQUARIUM salt per gallon of water in the tank. Do daily water changes.