My daughter is 11 years old and she has been having major mood swings, stomachaches, and headaches.?

Hormones. She could be entering puberty but it is always a good idea to have a doctor check out headaches that are new onset. Stomaches & headaches together could be migraines but all 3 symptoms could also be caused by too much sugar and not enough protein in her diet.
Dehydration. I agree with dr. Block's answer completely. One of the other potential contributions to your daughter's symptoms is dehydration. Teenagers are notorious for being dehydrated much of the time. This has a lot to do with their fluctuating hormone levels, and the typical liquids teenagers tend to imbibe (e.g., soda, energy drinks, etc.) which all contribute to dehydration.
She needs evaluation. She needs to be evaluated by a mental health professional.