Do I have an eating disorder?

So over the summer i became really focused in losing weight. I was not overweight or anything just chubby in some areas which really ticked me off cuz all my brothers ate a lot but look like skeletons. So i started gradually exercising more and eating less and healthier. Since school started though, I've really just become obsessed with it. Everyday i wake up in the morning literally worried that ill eat too much that day or won't exercise enough. I feel like every time I'm eating, whether its just my dinner or if im having a cracker for a snack, everyone is just watching me and thinking that I'm a fatass. I try to eat under 1200 calories a day, but i always feel as if i messed up and ate too much and if i eat normally then i get pissed off at myself and start exercising or I'll eat less the next day. I hate how eating makes me feel, and i rather be hungry than feel so guilty. I have lost a few pounds, and im not sure how much because i don't own a scale but now all my jeans are loose and i still don't feel good about my body. I really don't want to go back to my old diet, because every time i look back at what i used to eat im disgusted with myself about how much i used to eat. I hate feeling this way but i don't to risk changing and going back to being even fatter. Eating makes me feel so guilty- i'll eat a cookie but later won't it go. Since i've been eating less my digestive system got messed up and i haven't had bowel movement in over a week. My family keeps making fun of me that i have an eating disorder but is it that or am i just dieting???

Im sorry i understand how stupid i must sound but im really just sick of this and any help will be much appreciated.