Sorry I only just noticed this thread now. I hear you totally on feeling/looking fat at at 21 BMI. I'm about that at the moment. I know I'm not fat or overweight... all I know is that I feel and look gross, and my clothes are too tight! So, don't stress, you're not unusual, and you're not alone!
I would say definitely seek some professional help. As to which kind of professional I can't say. Do some research on the net, but whatever path you decide, make sure there's someone watching out for you, just in case the anorexia rears it's ugly head again!