I have a lump at the back of my kneewhat would it be?

Baker's cyst: A very common occurrence as part as knee problems is a Baker's cyst which is a fluid filled lesion that grows in the popliteal fossa or as you said back of the knee. It could be very painful especially if it breaks. If small you can treat it with warm compresses. If too big then should be evaluated for possible drainage. However, in order to have a definite diagnosis you should get seen.

Bakers cyst?: A common cause of lump behind the knee is Baker's Cyst (a pouch protruding from the knee joint filled with synovial fluid) See ... http://patient.info/health/bakers-cyst-leaflet ... for more information, Other rare causes can include clots, swelling of blood vessels, or soft tissue tumours. See your GP routinely to confirm the diagnosis.

Lump: Bakers cyst is most likely possibility. Other possibilities could include lymph nodes, abscess, boney growth. wil need to see doctor to assess and investigate. http://patient.info/health/bakers-cyst-leaflet