Hi, I have a teenager who suffers from anxiety. You may want to look into that. She would lay in bed at night and worry about so many things. She would worry about big things ( how come kids in Africa sometimes can't have clean water) to little things (what if my best friend isn't at school, who will I sit with at lunch). I found that having her talk about things that were bothering her before she went into her bedroom seemed to help. I tried to encourage her to get all of the worry out before bedtime so she could sleep better. 

I had that happen with my 11 year old, but only at certain times of the year.  After checking with the pediatrician, we came to realize that seasonal allergies made it so she was a bit lactose intolerant.  So in April and May we really cut back on Dairy with her and it's helped.

I wouldn't have thought it either, but the fact that it's happened two years in a row at the same time definitely helped proof it.  I think the extra congestion from the seasonal allergies combined with the milk to give her severe stomach pains.  Cutting the milk really helped.   She was still able to eat yogurt.

When I saw this question, "anxiety" was the first thing that I thought of. I have an anxious girl who often has physical symptoms of anxiety and was able to recognize them because I had the same problem when I was younger. 

I would also say maybe anxiety or stress. I had problems as a child due to both,,which improved after my parents divorced. They even tested me to see if I had an ulcer when I was young.

If the pain's chronic & repetitive I would mention it to her doctor at her next visit. I think it's important to rule out health problems.

I think unhealthy eating could be the cause of various stomach issues.