Some time I get a short thobbin pain in my right knee it appears swollen some time with a small knot in the back of the knee?

Bakers cyst. A bakers cyst can be found behind the knee. Half the time it is associated with a torn cartilage in the knee, half the time it just happens. It's like a hernia of the joint capsule in the back of the knee. Generally it gets larger when you're more active. Most are not painful unless they are very large and are pushing on something else or rupture. If it keeps bothering you, see a doctor.
This may be a cyst. Possible a baker cyst. Need an exam to decide on further treatment.