Hi there, Mousie. Welcome to the boards!
I agree that you should get a medical professional's oppinion.
One question - when you say that you are always eating, does that mean because you're always hungry, or is it more because you get anxious when you're not eating.
If you are NOT hungry and you are still eating, there IS still a posibility that you have an overeating disorder. The fact that you are underweight could STILL be because you have a hyperthyroid disorder or an overactive metabolism, like an above poster has already said. I'm not saying you DO have an OE disorder... I won't say whether I think you do or I think you don't... after all, what do I know? You need to see a doctor about it to get an explaination.
Why do you get anxious about not eating? Are you worried about the fact that you are underweight? Is eating a way to stop those feelings maybe?
How about the amount of exercise you're getting in one day. Would you consider it moderate, little, or a lot? If you're over-exercising that could explain a few things! And what about the foods that you're eating? If they're mostly things like fruits, vegetables... foods that are easily digested, you may want to alter that.
Sorry about all the questions! I hope you get this sorted soon!