A sign of a problem elsewhere
A Popliteal or Baker's cyst can appear suddenly and be experienced as a painful lump on the inner aspect of the crease at the back of the knee. It's appearance generally reflects a problem within the knee, such as early arthritis, when there is excessive fluid pressure inside the knee, and the pressure causes a herniation of the joint capsule through an anatomical weakness there.
Baker's Cyst. Demange MK., Rev Bras Ortop. 2011 Nov-Dec; 46(6): 630–633.
Popliteal cysts: a current review. Herman AM, Marzo JM. Orthopedics. 2014 Aug;37(8):e678-84
Baker's Cyst Filled with Hematoma at the Lower Calf Yoo M-J, Yoo J-S, Jang H-S, Hwang C-H. Knee Surg Relat Res. 2014 Dec; 26(4): 253–256.