Hey there, Tabee.
If you're eating properly and all (three meals a day, most days, plus snacks if you're hungry, etc) then you probably do not have an eating disorder. That said, I'm not a doctor and cannot diagnose you one way or another.
Being thin doesn't mean you have an
ED, but it would be cause for medical professionals to wonder and ask. Because eating disorders are becoming more prevalent in our society, all medical professionals (including dentists) are (or should be) on the lookout for signs of their patients having an eating disorder. If you're eating in a healthy way and aren't purging or restricting, be honest with them about that.
If you think you have a distorted body image or an eating disorder, take advantage of this opportunity to talk to your doctor about this.
Let me know if you ever need anything, Tabee. I'm just a
PM away. =)