The most worrisome thing would be a popliteal vein DVT.
https://www.webmd.com/dvt/thrombosis-bakers-cyst
After that, it can go to a few less worrisome but still painful issues like hamstring problem, baker's cyst, sciatica, lumbar radiculopathy...all of them are varying degrees of the suck but are unlikely to evolve into a potential show stopper like a DVT->Pulmonary Embolism (PE).