Help!!! I don't know what's wrong!

Jun 19, 2008
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I have a female betta fish named Faith, her age is unknown as she was a Petsmart find about 6 months ago. She lives in a 2 gallon tank, alone, with a heater that's turned on when it gets too cold. Full water changes about every 2 weeks, give or take, and yup, i use aquarium salt (learned that after she had a fungus a while back).
I just changed her water today, it was probably over 2 weeks because I had to wait to get the water (we get it from our rental because we don't have to adjust the pH that way). I let it sit out for over 24 hours to get the chlorine out like I always do. But when I got her back in the tank, she was acting lathargic and just kinda sitting there.
When I checked her tonight, she was just floating at the top of her tank, not really breathing with her gills too much, but taking glups of air every once in a while. Not really what I would call her regular breathing. She would swim to the bottom, and then just float back up w/o any resisance. She started hiding herself under her plant and big rocks and not responding when I came near the tank (i'm usually greeted with an inquisitive look right up against the tank and a glare that says "feed me") Fins are crossed and clamped against her body most of the time. Her color is pretty faded too, she looked almost white around the head/ gills (she's always been a little lighter there, but not that much).
I did take her out of that water and into some other water from a different "batch" in a smaller container, and she seemed to be doing a bit better there.
From reading stuff online, it's either something in the water that bothered her for some reason, or swim bladder disease (which i was guessing because of the way she was floating). I could be the culprit here, I have a feeling I was feeding her too much and that could be the problem.
Any ideas, guys?