Could be a number of things...

-Seat height and/or position (probably not if your right leg doesn't hurt)
-one leg is longer/shorter than the other (probably not)
-Muscle imbalance or tightness
-Ligament/tendon tightness
-Foot alignment / float
-Overly flat feet or overly high arch
-Weak ankle causing your foot to roll in or out

Any of those seem to fit?