On my latest bike, I'm getting pain like a mild strain behind my left knee after a brisk 5 mile ride. The bike is a vintage tourer. It's probably a little small for me, but I can get the seat to what I think is the correct height. (knee slightly bent at bottom of stroke) The bike is a 56cm square; I usually ride a mountain bike in a 20 inch. This happened twice; after the first time I dropped the seat down about 5mm. Is there a problem with my form or is it seat height or am I bending over too much because it's a drop-bar bike?