First, it doesn't sound like you have an eating disorder-- but when in doubt, it might be good to find a professional to talk to about it.
I read a body mass index and it said that considering your height(5ft 8 inches) you are in the normal range for your weight (which is around 135-140 pounds-- converted from 10 stone-- hope I have that conversion right..??..)
It could perhaps be that your appetite is low when stress is high (I'm like that) or when depression sets in (I'm like that too). A lot of things keep me from eating, maybe you are like that too???
I don't know your age but when I was a teen-- all the way through my 20's I weighed around 98lbs (7 st) and I've not ever been anorexic--(though I was way underweight then) I never had the illusion that I was too fat--- just stress and depression keeps my weight low.
I hope this helped some and if you want I can PM you the conversion table site and the body mass calculator.
Wishing you a good day/evening.
mandy