How do i know if i have a serious eating disorder?

The fact you asked: Usually, if someone is wondering if they have an eating disorder, something fairly serious is going on. You don't say whether you are over- or underweight, but i'm guessing the latter and that people are worried about how thin you are. If you're convinced your "fat" when others are telling you otherwise, restrict food intake, exercise excessively, binge, or purge, seek professional help.

All ED's are serious: If you mean anorexia nervosa, it has a lifetime mortality (death) rate of 10%--that's serious. If you mean bulimia nervosa and binge eating disorder- though their mortality rate may not be as high, their potential for ruining a life and causing psychosocial misery and medical problems is very high.