I experience this from time to time. It's more common during the cold weather or after I've taken some time time off due to snow or bad weather. I usally ice down my knees for fifteen minutes after riding and if the pain is real bad I also take an anti inflammatory like advil, then when I get back on the bike again I spin lower gears till I'm loosened up. it usally goes away after a while. I think it just an over use kind of thing but its a good idea to have a doc check it out to make sure it is not something bio mechanical , torn tendon or the like.