it is because of two reasons one you are consumming more calories than you are burning and also you are eating the wrong kind of calories i would bet you do not want to eat what are called empty calories such as chips or other snack products. when you look at the lable one a product you need to make sure it has carbohydrates for energy because you will need it to keep working hard also you need proten because your body is probably not being able to repiar it self so it is not burning as much fat as you would hope.i would take the spped up on that treadmill run maybe some where around 6 mph or more because yours is fast walk thats all. i hope this help you and start to lose wieght.