Thank you for your query.
The knee joint is the most used joint in the body and is well equipped to handle the stress.
However, overuse, overweight, age related degeneration, injury and other coexisting illnesses may speed up degeneration of the knee. The joint is made of the thigh bone and two bones in the lower leg. This is surrounded by the joint capsule that keeps the knees well oiled.
The synovial space contains fluid that nourishes and cushions the joint and distributes any impact evenly. This area also includes nerves and blood vessels that supply and inner ate the leg

This ability reduces with age and factors mentioned above. Under these circumstances, when the knee is burdened by stressful movements, the impact is focused on certain areas and affect the surrounding structures. This can lead to straining a ligament traumatizing the joint.
The cracking sound results from joint damage and bone rubbing over bone during a movement
The nerve that sensitizes the knee also innervates the lower leg and toes. Certain muscles and tendons extend from your knee and down your leg. Injury along this area can lead to inflammation and pain.

I recommend ice compresses, topical anti inflammatory application twice a day,
rest and limb elevation if there is swelling.
Remember to ice the area for no more that 10 minutes (on and off - don't leave the pack directly on the skin). Try to walk normally (don't limp) as much as possible.
If the pain doesn't reduce in two days of trying the above, or you have severe pain/restriction of movement, see your doctor for to rule out severe injury

Hope this helps. Wish you good health.
Please feel free to contact us if you have further questions