Do I have an eating disorder?

My sister says I must go to the doctor with my eating disorder.

I’m 19 and have been struggling with my appetite lately. I’m at 125 pounds now, and my goal weight is 110 pounds. I lost about 12 pounds in the last three months. I usually weigh myself daily, and my belly fat has almost disappeared visually. I’ve never made myself sick on purpose! It’s true that I used to be happy when I had a stomachache.. I thought that eating less would make me skinnier in a short period of time. But I don’t have such stupid thoughts anymore, in fact, I’m closer to my goal, and it makes me happy! I restrict myself to 1100 calories per day. It’s not that hard, I just have to skip breakfast and eat less in the evening. Though lately I’ve had issues with evening meals – I don’t feel like eating. I thought it was due to my illness, but the appetite never improved with my recovery.

I often argue with my sister, who likes fatty foods and insists everyone should eat the same things she likes! I wouldn’t be asking for a second opinion, but I had a delayed period for the first time in my life last month, and I’m a bit worried about it. What doctor should I visit just to make sure I’m all right?