Hi, thanks for writing to HCM.
Based on the description, I suspect that the swelling at the back of your knee could possibly be a BAKER's CYST
. This is commonly seen in osteoarthrtic knees and usually results from excessive bulging of the synovial reflection along with fluid into the back of knee.
This is generally harmless and in ocassional cases may limit joint motion (if it is too large).
* a short course of an antiinflammatory medication may be helpful
* Aspiration of the cyst will ease pain & improve joint motion
* Injection of corticosteroid into the cyst will benefit in some cases
* Ocassionally, excision may be indicated
Hope this information is helpful. Good day