Why do I feel guilty after every meal?

I have no idea what is wrong with me, i've been dieting for around a year? I've lost nearly 3 stone (around 40 pounds i thing) but after every meal, healthy or not i feel unbelievably fat and discusting, i feel like i want to stick my fingers down my thoat and throw everything up or excerise for hours! Like today i've had so far
breakfast: chopped up green apple, chopped up banana and natural fat free yogurt
but i felt fine after that.
snack: i had a flapjack because i baked them and i feel Really Really fat and that i shouldnt eat for the rest of the day and i've already been for a 1 mile jog, cardio excerise like jumping jacks and squats, press ups and crunches
i'm going to do a zumba work out soon because i still feel horrible! Sometimes even if i have an apple or veg or fish i feel really fat, what's wrong with me? I'm not going to eat any lunch because i feel disgusting about myself. Also i use an app on my iphone called myfitnesspal.com and it lets you count all your calories and i have a goal of how many calories i should take in 1 day and i've set 800 for me, is that healthy? But i normally only have arounf 450-500 rarely more. I just wish i was like all my friends who can eat candy and cookies and chips and not worry about there weight. Btw i'm 14, 5ft and i weigh around 138 pounds.

You apparently have the beginning of an eating disorder, which is a psychological issue, not a physical one. Unless you can stop obsessing over calories and stop feeling 'fat' every time you eat something, you are endangering your health. My advice? Speak with a therapist or counselor, preferably one who is a specialist at helping with eating disorders. The sooner you do it, the easier it will be to deal with, I promise.

If you can't eat enough you can go to the food shelter

Probably because you have built up a habit of doing it. The feeling might come now, just because it is primed by the cascade of events: it happens when the meal is over. When you notice the feeling, try and just look at it like a thing in your body. Don't try to make it go away or anything, just let it be and kind of study it. Ask yourself if there was a trigger moment that happened just before. Was it seeing your plate almost empty? Was it when you considered taking your last bite? Ask yourself do you really feel this way, or is it just the song playing out as it always has, this is the second chorus. This will take repetition and practice. When I used to smoke, I just thought the craving for a cigarette came when it did. After a couple weeks of studying the feeling, and what surrounded it, I realized that there were things that came before the initial craving. Frustration was understandable, but I was surprised to find that boredom was almost as common. When I had the feeling "I'm bored," it was often almost immediately followed by a craving for a smoke. Once I knew that, and watched it play out, I saw how mechanical it was. It made the craving less severe because I knew it was just an automatic response to the feeling of boredom. It took a while, so be patient.

800 is way too low especially since you're still growing. You're depriving your body of necessary nutrients. You shouldn't go below 1200. You're working your way into being anorexic between severely restricting food and over-exercising. Look at a BMR calculator to see how many calories your body needs with no extra activity and go from there.