Racers who work out improperly with weights usually end up in your situation. Most males will work their upper body more than their lower and this will work against you on the bike. you will start to ride the bike with your upper body and not your lower. I would suggest that you start working your lower body and core(abs/low back) for strength and use muscle endurance/plyometric exercises for your upper body. you may have to slow down for short period of time to allow your body to compensate for this different riding style.
Alan