I'm a 17 year old female weighing 8.5 stone and I'm 5 foot 5 inches so I would think of myself as a healthy weight however I am having a lot of problems with my appetite. For long periods I will feel really full and sickened by the thought of food and then within seconds go from being really full to so hungry I feel I could devour a whole cow and then I'll go back to being really full and sickened by food as soon as I start eating. I don't understand what's going on it's completely messing up my eating habits does anyone know why this is happening I'm not pregnant ?

You're going to need to see a doctor. You could have an ulcer or some other medical reason that you cannot tolerate food well. Most anything it could be can be treated effectively, so tell your parents you are having this problem and ask them to take you to the doctor. Tell the doctor everything you can about what has been happening and don't worry about embarrassing details if there are any - doctors hear it all and they need to know all there is to know to make a good diagnosis and get you the right treatment.

It could be something like a food allergy, so that when you eat certain types of things you can't tolerate any more food. Some people have things like intolerance to dairy or gluten, for example, and can have the kinds of problems you describe.

You won't be able to solve this one without some professional help, so get it so you can start feeling better.

Okay thank you for your advice I shall see a doctor about it asap :)

It could also be extra steep hormonal fluctuations. Testing them is expensive, and your doctor may not be willing to do it.
I think I have gone through this, and I'm pretty sure it was related to my menstrual cycle (years ago). Pregnant women go through some of the same wild swings of hunger vs. almost nausea at the thought, sight, or smell of food.
I would keep a notebook of what happens when, and keep track of your periods too, and see if there is any correlation.