Pre-patellar bursa
Housemaid's knee is the same thing as 'pre-patellar bursitis'.
The bursa may fill with fluid and become red and hot, and painful when the patient tries to continue the activity that triggered it off.
"....A bursa is a fluid-filled structure that is present between the skin and tendon or tendon and bone....The main function of a bursa is to reduce friction between adjacent moving structures...."