Could be any number of things, gatiger, but this is my best scenario:-
When tissues have been divided or otherwise interfered with, or stretched in an abnormal way (this happens during surgery) they are naturally bruised and contused as a result. Now if you think about a more normal situation - perhaps you have had a fall where you grazed you knee, got bruises or pulled a ligament. Afterwards does it not feel constrained and stiff? In other words - it hurts!! But the mind plays tricks and interprets this into things such as you describe like the rubber band feeling when in actual fact it is simply pain and needs dealing with the same according to intensity. People who have had lesser ops like rotator cuff repairs or even as simple as carpal tunnel decompressions suffer the same kind of illusions. Even the pain from a tooth extraction can make you imagine a clamp on the side of your jaw.
So, provided the knee is not visibly swollen, or that you have pain and tightness to the degree that movement is almost impossible or intolerable, then simply use the tools you have at hand - elevation, ice, TENS, meds - to get it under control if you feel the need.
Hope this explains it a bit.