That's a pretty small tank. In a tank that small, you should definitely be doing more water changes. Once a week is not enough.
You should think about getting her a bigger tank and a heater. Bettas are tropical fish and like warm water, around 80 degrees. The water in her tank is a few degrees cooler than room temp and because it's a small tank, the water temperature is more likely to fluctuate.
She probably lost her stripes after you changed her water because, number one her water was clean, and number two, the light probably provided some heat.
She'll probably unclamp once you start cleaning her water more and keeping her warm. Oh, and if you get her a heater (and thermometer!), she'll be more active.
If you don't think you can do as many water changes as are necessary with a tank that small, you could always get her a bigger tank with a filter.
Last edited by fidget; 06-02-2014 at 06:23 AM.