Basically cycling a tank is when you build up colonies of helpful (and essential) bacteria in your tank so the water stays safe. I hate to push the water conditioner since I know I've already pressed it, but It's something you'll really need. Since your fish is already in the tank, you're stuck with a fish-in cycle. You can read more about that here: http://www.bettafish.com/showthread.php?t=47838
*laughs* taking care of fish is way more hard than I ever expected it to be! Worth it in the end, in my opinion =)
Anyway I don't know if cycling is the problem here, but it's certainly something to look in to. I don't see anything wrong other than than that that you're doing, warm water, good food...we may need the help of OFL here, since I'm not a pro.