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.