If she’s having stomachaches, I would check for food intolerances or allergies before jumping to eating disorders. It sounds like she’s realized that maybe she feels better when she eats healthier foods and that rich foods upset her stomach. I’m the same way; I later realized that I really can’t eat foods with dairy without feeling sick afterwards. Even as a kid foods like pasta with Alfredo sauce were too much.
Definitely give your ped a call, but I would probably rule out food intolerances and allergies or possible reflux first.

I would check in with the pediatrician, just to be safe.

OP, will you have some uninterrupted time with your child over the Thanksgiving holiday?

Observe her for all her meals Wednesday or Friday. It is easy to lose track of what your kids are eating when they are teens because why should you have to monitor? So watch her without her being aware -- is she skipping breakfast, nibbling on some crackers, having a light lunch "because my stomach hurts" of an apple, and then eating a normal sized "reasonable" dinner?

OP, yes, this could be anorexia.

If she has actually lost weight (and it seems you have reason to believe she has -- loose jeans etc) it isn't just the start.

The sooner you can get treatment, the easier it is to turn this around.

I recommend you take her straight to a good eating disorder clinic. Are you in the DC area?

PP's advice above is perfect. Educate yourself about this now.

A few of us here gave this advice to another parent (stepmom) 2 years ago if you are interested in reading the thread:

http://www.dcurbanmom.com/index/posts/list/663477.page

It is so hard to know what to do, and you don't want to overreact, but if it is anorexia you want to react very quickly and decisively because the sooner you turn it around the better.

This is how our daughters eating disorder began. I pray this is not the case for you. If you take nothing away from my post, take my advice to post your concern at “F.E.A.S.T Around the Dinner Table” forum. Those parents have been through this and know what it takes to help a child recover. I followed their advice and was able to help my daughter recover over a two year period. She is 23 now and doing well. Have hope!

Our daughter only lost about 7-8 pounds before we took her to the doctor. She never lost more than that because we got an early-ish diagnosis and began to treat it. But getting those lost pounds back on her and then additional weight commensurate with her height gains was a huge challenge because the eating disorder mindset was quietly but FULLY entrenched even with a loss of just 7-8 pounds. She still loooked healthy and not too slim. I learned that kids gain several pounds of weight for each added inch of height: new bone, tissue, blood, etc. On a normal course, they do not lose weight as they get taller. She was losing weight and not on a normal course.

The advice given to me was to make an appointment with the pediatrician and ask for the nurse to call you ahead of the appointment because you have a concern. You don’t need to have the doctor to call you ahead of the appointment. The nurse will call and tell her that you are concerned. Ask if she can weigh your daughter without sharing the weight with your daughter when she comes in for her appointment. She will have your daughter get on the scale facing the nurse and the daughter won’t see the numbers. The nurse can deflect your daughter’s interest in knowing her weight. Our nurse did this very naturally and was subtle in quickly getting a weight without my daughter seeing the weight. My daughter always wanted to know if her weight was going down. The eating disorder mindset finds that information reassuring so by weighing her backwards and not giving her the information, it did not feed her compulsion to get lighter. This was just one tip that we got from the around the dinner table forum. It was very helpful. We got rid of the scale at home and she was only ever weighed at the doctors office during that two year period.

Start with the doctor but we warned that your doctor may not give a diagnosis of an eating disorder at this point because there may not be enough data yet. Proceed to arm yourself with information (some doctors are not very good at diagnosing this or knowing how to treat it and they may lecture the child on nutrition or think that a pep talk will work. Lectures will NOT work).

Once our daughter got the weight back on, the mindset receded quite a bit and she ate more normally and her mind and body seemed to be in less internal conflict. We ate three meals and two snacks with her a day and the weight went slowly on her. It was not fun.

She talks now about that dark time and doesn’t remember much, but she does say that there was a voice in her head telling her she was bad if she ate and good if she starved. The voice was verbally abusive to her and frightening. It breaks my heart to know she went through this.

She was later diagnosed with high anxiety and some depression. The anxiety was primary. She benefited from counseling to address those issues which preceded the eating disorder and were still there afterward. I do wish we had added an anti-anxiety medication in that first appointment with the pediatrician instead of waiting until she was 1.5 years into the eating disorder recovery. That would have helped immensely.

Hopefully, all this information is for naught and there is a simple explanation for your daughter’s weight loss.

I would check out her intake. It sounds like she is obsessing about weight. I would be concerned about anorexia.