Lump on my knee cap, what could it be?

Bursitis most likely: These are potential sacs (bursa) of fluid which are present where ever a bone is rather prominent, ie, under the skin. These sacs get swollen to protect the underlying bone; & are liable to get inflamed (then called bursitis).

Simple vs serious: Our bodies always have things that grow and change. Skin, fat, muscle, lymph nodes, bones, can change shape, get larger, more prominent due to many factors. The vast majority are not cancer but simply unusual growths like fatty tumors, cysts, skin tags, moles, etc. Most cause no symptoms and can be watched. If they cause symptoms or change colors, bleed, bruise or hurt, then see your doctor.