I recently adjusted my seat on my commuter bike to be higher, I like full leg extension on the down-stroke (though LBS said this could be problematic?), and I like to be as high up as possible to see and be seen by motorists. I was also kneeling on the floor recently, to work on a bike I think, possible other explanations I just can't recall ... I'm leaning to this being caused by kneeling (prepatellar bursitis) ... could the seat adjustment have caused my one swollen knee? Other knee seems OK.

Sharp pain under the knee is Saddle TOO High.

You don't say how much higher the new position is compared to the old position. Sometimes too radical a change in saddle heigth can cause knee problems. The other thing that comes to mind is working on your knees on a hard floor. I know this from experience. If the problem does not clear up on its own in i week or so, I'd then begin looking at the position on the bike.

On second thought, if you have recently increased your mileage by a significant amount, say, 30% or so, this could also cause knee problems. Good luck with the diagnosis.

can't help diagnose that, sorry. but last spring, I sprung for some cheap Home Depot knee pads so I cud do some stuff around the house

Was it the front or the back of the knee swollen? I had the back of my knee swollen, and I believe it's caused by seat too high and riding too hard. Like you I like to have the seat as high as possible and the knee fully extended, because in the past I had knee problem due to seat too low. But this experience told me the seat should be neither too high nor too low. I make it high but not too high, knee still slightly bent at the lowest pedal cycle.

Thanks all. I went to the doctor. Knee issue was probably unrelated to seat height (though I may still lower slightly) and more likely from kneeling to work on bike!

My own Google diagnosis (based on pregnant appearance of knee) was correct. Now using pillow to kneel until I can afford good stand (maybe will try some pads!). I had an Xray which also showed very slight, normal-for-my-age arthritis.

The knee swelling was an unsightly thing called prepatellar bursitis - with no pain - only mild discomfort due to swelling - the doctor stuck a needle in and drained a few ounces of blood-tinged fluid into a syringe, and injected a steroid using same needle, so only needed needle to go in once, looking better now. Ideally the fluid would have been straw color synovial fluid, blood indicates trauma which I don't recall, except for kneeling, which can inflame the bursa to react and cause fluid buildup and an inflammatory response, strange.

As you recover, try to spin more than mash...use an easier gear and keep cadence up. Otherwise, you can cause a recurrence. Ice after you ride, as well. Bursitis can flair up again without much trauma. Been there. Good luck.