Hi there If the swelling behind your knee is not pulsatile and disappears at times, it is possibly a bakers cyst. A bakers cyst is a benign swelling of the synovium of the knee joint. it is usually associated with arthritis of the knee. You can reduce this swelling by resting your leg, elevating the leg for atleast half an hour a day, using ice to the affected area and taking anti inflammatories .. If the swelling does not resolve with the above treatment then you have to visit your doctor to do an ultrasound and X-ray to investigate further. Hope this helps.