you are definitly not fat. and yes, it sounds like you do have an eating disorder. if you are a cheerleader, then you probably have alot a muscle, and muscle weighs more than fat, so keep that in mind. you probably have more muscle than much of your family. other than having an eating...
if that is your only symptom, it could just be that maybe you been exercising more than usual or something. i dont' think there are really any health conditions that you have to worry about, but i would suggest you go to the doctor just to be on the safe side. good luck
postpartum can last for 2 years after delivery. but you could also have depression that is not postpartum. you should talk to your doctor. good luck and congrats!