This can happen when a bulb is burnt out, or if the headlamp is not making a good connection with the headlamp receptacle.
I had this same problem, fast blinking signal, when replacing my front headlamps - I had to hold the headlamp housing in place from under the wheel well to make my new lights fully connect when pushing them into place.
I would check visually to verify all lights are working as expected. Let us know what you find.
