I have always been overweight, but I don't eat more than 700 calories a day. I know that I should eat more, but when I do, my weight spirals out of control. Is there something wrong with my metabolism?
Seven hundred calories per day is really, really low. And it's unhealthy; there's no way to get enough nutrients on so few calories. In fact, it really takes at least 1,500 calories daily with carefully chosen foods and a multivitamin/mineral for insurance to get all the vitamins, minerals, and phytonutrients you need.
But if your metabolism is superslow, you can probably get away with consuming 1,200 calories per day, supplemented by a multivitamin and two 500 mg calcium pills. Your food choices must be high in nutrients, though (vegetables, a few fruits, whole grains only, lean meat, skinless poultry, fish, and healthy fats). There's no room for junk food!
What I'm guessing (and hoping!) is that you're underestimating your daily calorie intake, and you're really eating more. Try the 1,200-calorie a-day Boot Camp program in my first book, The Supermarket Diet. That phase runs for just two weeks, then you step up to the 1,500-calorie-a-day Keep on Losin' program. See what happens to your weight after that. I don't think you'll gain but, if you do, consult a doctor.