Large swollen lump on left leg just beside knee and reaches to bend of leg. Size of an apple. No injury and was sudden. What could this be?

See your doctor. You could have spontaniously tore a muscle or tendon, see your doctor for further evaluation.
Could be Bakers cyst. It's sounds like a baker's cyst - synovial sac of the knee joint can produce a posterior bulge, into the popliteal space, the space behind the knee. Most baker's cysts have direct communication with the synovial cavity of the knee. Typically arises in a context of arthritis or meniscal (cartilage) tear. Other less likely considerations include popliteal artery aneurysm, hematoma, or even tumor.