Hi,
I also think it may be anorexia. I struggled with anorexia when I was taking a certain medication- the side effect was anorexia! Well, I blame the medication for disordering my eating at that time, but I'm still terrified of gaining weight, since I used to be heavy. I'm the same height as you and I weigh around 110, so weighing under 100 pounds is definitely VERY thin, not to put you down in any way, shape, or form. I personally think you should seek therapy because you do not want this to get out of control. I no longer have disordered eating, but I have a distorted body image, and I do know what it's like to be like you. For me, it started out the same way. A little voice would tell me not to eat something fattening, too, and then it got to the point of obsessively writing in food diaries and counting calories, using diet pills, printing pictures of emaciated girls out and making collages, pinching fat pockets all day long (I STILL do that), and exercising excessively. Of course, you may never get like that, but if you are concerned that this is affecting your life, please, please, seek therapy now. I wish you the best of luck.