Why have i had a lump under my knee for 2 years? what do i do?

See details: Swelling in the back of the knee often is the result of a popliteal cyst (baker's cyst). If it is not causing problems it often requires no treatment. See an orthopedist if you are concerned.

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.