if its as tight as you can get it, (the axle does not move) then it will be your cones. if not, like the other fella said, it will probably by the frame twisted or cracked making the wheel move when pressure is applied to the pedal arm causing it to move over to one side as most have you will have experienced? apart rom that it may be the axle snapped or twisted, like the other fella said. i had this problem once but it turned out to be that my hub had cracked and was about to cave in! your best bet is taking it down to your local bike shop and letting them have a look at it. Tom