Its due to osteoarthritis of the knee, an old ageing problem leads to knee pain, swelling, muscle weakness and stiffness specially during sitting to standing/squatting /walking /stair climbing.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy +IFT +wax therapy +some strengthening exercises will be helpful for sure.
Health Tips
At home.. Rest +hot water fomentation +pain killer gel +no long standing /walking /stair climbing +no squatting +no low sitting +no cold exposure +do cycling+ use knee cap .