Thanks for asking me to respond.
It's possible that sitting in the same position for a long flight can cause stiffness in the knee. Stretching out the knee should reduce the pain slowly over a period of 2 to 3 days. If pain persists beyond that, you will have to visit an ortho surgeon who will examine the knee and order xrays of necessary.
Other common cause of knee pain at your age is a meniscal year. Stiffening of the cartilage behind the knee cap can also cause similar pain.
I cannot prescribe meds as Practo prohibits me from doing so.