I have no idea, sorry. Nothing in my experience. Only think I can think of right off is that your cadence may be too low. 90-100 seems best for folks with knee problems. Cyclometers with cadence display are inexpensive. Cateye Astrale is a good one.

I'm a little suspicious of doctors who say nothing is wrong. This is not normal, in fact it is extremely unusual. I've had doctors tell me there's nothing wrong when I've had a serious structural defect that needed to be taken care of, only convincing them to try for a diagnosis after months of pain. In the US it's common for doctors now to go to great lengths to avoid spending insurance money.