What is the make of your implant? Zimmer seem to be having loosening problems with their NexGen CR Flex implant, especially when fitted using the minimally invasive technique.
If you look up oohyeah4 on this forum (2 and 3 pages back) his name is Rob, he has had loosening of the implant (Zimmer) and has since had a successful revision (using another Zimmer implant), you will learn a lot from his experiences.
As for the pain behind the knee cap, I have had ongoing pain for 3 years after a right TKR (Zimmer NexGen CR Flex), I too thought that the implant was loose because I had pain after standing, especially when lifting my leg after standing or walking, I eventually had a arthroscopy which revealed that I had grade 3 OA on my patellar. I have just had the patellar resurfaced and I am praying that it will be the end of the terrible pain, time will tell, but the thing is, I could move the knee cap around OK and even kneel on it OK without pain (You can get referred pain from anywhere in the knee joint, even from your hip or back) huntingandmoon, Diana, found a great surgeon that used a anaesthetic in her knee to eliminate referred pain brilliant yet so simple.
Good luck and I hope that you find a solution soon, it can be very, very frustrating.