Also check the height of your saddle, and it's position relative to BB. If you are going to get in to it regularly might be worth it to get a proper fit.
Are you using clipless pedals? Check to see where your toes are pointed. They might be too much in or out on left side and it's putting tension on the knee.
Also if you are using clipless it's never too early to practice proper pedalling technique. Find a quite road, shift to easy gear, unclip one side and pedal with one leg then switch. You are looking for a smooth motion.