Here's my $0.02...I had a loose cover on my rear bulbs. You know how the rear cover plate with the bulbs in it? Well that round connector that clips into the turn signal harness was a little loose. Some dielectric grease helped a little, but I think I'll be in there to crimp them back together. It only happens when it's very cold or very wet.
Took me forever to track it down. I put the blinker on and then twisted the release knob until the fast blinking went away.
Alternatively, it can be dirty connectors/internally cracked or chafed wire/the wrong wattage bulb. Believe it or not, the wattage can effect the blink rate. But I'm thinking the above solution is where your trouble is.