Welcome to HCM.
Most likely it is due to muscle sprain
There is over stretching of muscle with unequal body weight on it.
Apply ice 3 to 5 times a day.
Intra muscular injection of nsaid is advisable if pain is severe.
Apply 4 inch elasto bandage on it during movement.
Avoid any extra weight bearing on it,Keep your leg raised as much as possible.
Consult your orthopedician for proper diagnosis and medications.