Hi, and thanks for your medical question on eHealth forum!
Well, the lump you have could be a cyst and as is more towards the back of the knee, then it can be a Bakerâs cyst or a popliteal cyst. The other possible cysts are sebaceous cysts which occur due to blocked sebaceous glands.
The other possibility is a ganglion. This can arise due to repetitive use of joint or due to rheumatoid arthritis or sometimes without a cause too. It is usually a swelling in the tendon of a muscle. Since you are walking a lot now, this could be a cause.
It can also be a swollen lymphnode if you have an injury in leg or a skin rash or eczema near by.
Since a confirmed diagnosis cannot be given on net please discuss these possibilities with a doctor. Hope this helps. Take care!