Hard lump on left patella that's painful when I kneel down and come back up?

Bursitis . This may be an inflammation in the little sack or bursa just over the patella. It normally helps everything "slide" when you bend your knee, but can get irritated, especially if you did a lot of kneeling or caused some inflammation to start. If it is red and painful, it may even be infected so you should get it checked. Try an antiinflammatory in the meantime. Try not to kneel until it is healed.
Prepattller Brusitis. Plumbers, carpet layers, and other people who spend a lot of time on their knees often experience swelling in the front of the knee. This problem could be avoided by using pad when kneel.