Thank you for your query.
at this age can be due to:
2. Endocrinal causes.
Your daughter needs to be evaluated and her nutrition discussed with her so that the following issues can be addressed and resolved:
1. Weight and along with it the back pain
and her feet issue.
I suggest that you take her to your doctor who shall do the needful.
I hope I have answered your query to your satisfaction.
Wishing your daughter an early recovery.