has she started with her periods yet ?
Since she is so young, I would think H pylori. it is quite common and easy to get - just a little germ from not washing the hands properly. She might need to take antibiotics.
Yogurt, acidophilus, vitamin C and aloe vera juice should be soothing.
A doctor once told me 'think horses, not Zebras'. Let's start with something benign.
Could your daughter have trapped gas or be constipated?
Secondly, she's a bit young, but gall bladder can cause upper chest area pain and abdominal pain which can last 5 minutes to hours and hours. It's an awful pain.
Ask about her bowel habits and if that's all fine, I'd see the doctor just to be safe. It could easily be pulled muscles but if you can even do a walk in clinic, they can rule out anything serious.