Left Turn Signal Blinks Fast. Right doesn't.

Yesterday I noticed that my left TS was blining fast and groaned to myself
thinking tha tI had to change out the bulbs. I made sure to do a funtion check
of all the lights and the left rear TS was the one not blinking. But the nught
running light was on. Went to Adv Auto and bought a replacement pair and
when I pulled the "bad bulb" out I noticed that here wasn't anything wrong
with it. and just to check I swapped it with the righ TS. The bulb is good.
greased up the new bulbs and they work also. Placed a new bulb in the left
TS socket and still had the sam problem. What is the my issue since the
bulbs are all good?