2004 Chevy Impala Turn Signal

My left turn signal indicator in the dash flashes unusually fast, the left rear turn signal does not flash but everything is lit up in the back. There is only one bulb for the rear left tail lamp and turn signal. So if the tail lamp is lit up but the turn signal will not flash, then more than likely the bulb is not the problem right? The right side works fine, and the left front turn signal works fine, its just the left rear turn signal thats not working