At her age, the most likely cause would be a mld injury to the sensory
branch of the radial nerve. Where that nerve comes around the forearm and as
it proceeds up to the dorsum of the thumb, it's relatively vulnerable to
even minor trauma. It should heal, but may take some weeks.

Also could be a nerve root compression issue in the neck. If it doesn't go
away after a few weeks, she should see a doctor.

HMc