Personally, as a female, I feel good about myself when I know that I look good. I am also not really into the club scene, but when I do go out, some days, I feel like dressing casual but nice, and other days, I feel like dressing sexy. Unless you have some other reason to suspect that she's going behind your back with other guys, I would assume that she is doing it for herself.
It's also probably a good idea to talk with her about how you're feeling...maybe she can give you some insight into it that will allow you to stop worrying so much!
Negative Ghost Rider...the pattern is full.