Are you under a lot of stress? That can make you gain a lot of weight fast!
Because you hardly eat. Your body is in survival mode and turns whatever you eat into fat.
You probably consume more calories than you think, also carbohydrates (bread, cookies, etc.) and processed food (especially those containing HFCS . . . sugar content) are rampant in America. Fast food and soda doesn't help but, if you start to read labels, look at portion size, start exercising regularly and keep track of your real caloric intake you may start to see changes that are for the better weight-wise.