What Causes Knee Pain Despite Having Physiotherapy?

approx 12 wks ago I had left knee surgery (torn medial and lateral meniscus, chondroplasty, synovectomy) after a work related injury. the knee has been slow to recover. Steroid injection helped. pain resumed when it wore off. 2 days of IV solumedrol & then PO steroids x8 days (for bout of asthmatic bronchitis) also helped knee. now that has worn off. yesterday PT noticed lump behind knee (it was also painful to touch) - ultrasound revealed popliteal cyst has doubled in size in less than a month. knee also buckled on me yesterday for 1st time since before surgery. Flexion of knee painful. walking up stairs very painful, walking down less so. I was almost to point where I thought I could return to work-last week had no problems going up/down stairs. was nicely progressing in PT. now have taken giant step backwards it seems. No new injury that I was aware of. any ideas what happened? how can I treat myself (I ve been icing several times daily). still doing my PT as possible without incurring pain.