Do i have an eating disorder HELP!!!!!!?

I'm 16 years old, 5ft 6 and weigh 7st. I have always been thin compared to others, but i am suffering from depression and have also noticed changes in my eating habits which i know is normal for people with depression, however recently i've noticed that i notice calories and check how many calories are in foods. I constantly pull at parts of my body that i think are fat. Also people tell me that im thin but i notice the slightest problem in my weight. E.g. i think my thighs wobble when i walk.. i mentioned this to one of my friends and she told me not to be so silly but i really do believe that they do and its horrible. I am determined to lose weight so that i have no fat on my body. My periods are irregular however i have them at the moment... last week i was eating alot, which was unusual for me, but i know people sometimes do this leading up to their period. I felt really guilty but for some reason i couldn't control it like i usually could. This only lasted for about 5 days and now i feel the same as i did before. I've only lost about half a stone in the past 10 months, however i have grown taller so probably should be putting on weight.. but i really don't want to. I can't. I feel really scared when i think of putting on weight, i would feel really horrible if i stepped on scales and realised i'd put on weight. Do i have an eating disorder? or is it because of my depression. What should i do?