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There are multiple reasons for the pain you are describing. It's not possible for me to make an educated guess. Obviously you would want to make sure her electrolytes, including vitamin and mineral levels are not deficient. This could also be an over use or possible injury. If there is swelling or warmth it could be an arthritis, including gouty arthritis.
If it continues I would certainly suggest she consult her PCP. I wish you both well and hope you will find a solution to the painful condition.
Best of Luck,