Pes bursitis (more specifically pes anserine bursitis) is a painful condition typically related to over use.  It usually occurs after a change in training, such as increase in mileage from jogging, walking, or hiking or an increase in intensity of training.  It can also occur after a total knee replacement as the muscles that have been accustomed to a knee that has developed a deformity due to arthritis is now straight again.  The pain is on the medial, or inner, side of the knee about 2-3 inches below the joint.  It is an area where three tendons converge to insert on your tibia, or shin bone (these muscles act to flex your knee and adduct your leg).  The bursa acts as a protective structure to allow smooth sliding of the tendons over the bone, but it is also the bursa that is the cause of the soreness.  The treatment is stretching, message, and backing off the offending activity.