demie MM, when you said this yourself,

"我的体重也常常7，8磅的变化，很奇怪"

I think you do have a problem with your weight. When you are depressed, you can't eat for two weeks. That would slow your metabolic rate. That's why it is easy for you to gain back the weight once you resume normal food intake. Like other MMs said earlier, you need to excercise to tone up your body. Your weight may not change, but your body should look fitter.

Another point about "not having a waist". Some girls simply have a shorter waist. Excercise may help you define your waist, but don't expect the result to be dramatic.