If there's no history of injury or known severe arthritis, pain behind the knee is often associated with prolonged overuse or maintainance of one position,which may even be due to prolonged sitting.
Most of the cases recover well with basic home advice.
Next Steps
Consult a Physio Online or at clinic.
Health Tips
Change sitting position from time to time. Continue basic mobility but avoid long walks or run till advised by a Physio.