It is a dangerous path you are on and not only is everybody else telling you so, your body is as well. Losing your period is one of the clearest signals that something is wrong. Your body is crying for nourishment, and I'm sure you feel it.
The good thing is that you can grab ahold of this before it gets worse. I was the same weight as you are now when I really started getting scared; my heart rate slowed, my hair started falling out and my personality was wasting away with my body. Anyone who has battled anorexia or any eating disorder can warn everybody else until their blue in the face about how deadly it is, yet some don't believe it until they experience it themselves. Please listen to what we, as well as your body, are telling you. If you're not comfortable at 128, then find a weight you are happy at. As long as you are healthy.
Most importantly, get to know the person underneath. You sound like a smart girl. You know you are more than your appearance. Focus on who you wanna be, not what you wanna look like.
We're here for you!
Yes, you are definitely on your way to developing an eating disorder. It isn't how much you weigh that determines if you have an eating disorder, but how you feel inside. You look at yourself and you see fat when everyone else sees skinny; you obsess about weight and food and you have poor self esteem. Unfortunately, eating disorders tend to get worse not better with time. I strongly encourage you to ask your doctor or a school counselor for a referral to a therapist experienced with eating disorders so you can get this under control, get into recovery, and enjoy your teenage years instead of spending them obessesed with weight and miserable. You also need to be concerned about the effect on your health. I hope you find recovery soon, it will change your life for the better.