To explain the above a bit more - it's not because you cycle that I am saying that you are weak, but I am an avid cyclist too and the tendency is for the knees to point inwards towards the top tube, thus shortening the adductors in the hip. This is coupled with then glute med being on a constant stretch, and shortening of the hamstrings which can again lead to an adduction of the leg (semi-membranosus and tendinosus). In short, it is pretty hard to speculate as to what is going on without seeing you moving, but hope all this helps a bit!