Sounds like you have developed some patellofemoral symptoms due to the guarding reactions around the knee from your operation. Your physio should assess the mobility of the patella and treat it accordingly. This may have happened because the muscles have got very tight especially the quads and hamstrings. You will need advice from your physio or surgeon as to the correct time to start stretching because sometimes the graft is shorter than ideal and 6 weeks is still early days.
Its still fine to be using ice packs to relieve pain ie do your correct exercises and then rest with ice to allow any inflamation to settle.