Hello M,
First we need to know what truck you have. It would be easiest to go to the User Control Panel (UCP) and put your truck info in under "Signature". That way you won't have to type it in everytime you post.
First thing I would do is check the bulbs and the flasher.
Does it do it on both sides or just one? Does it do it when you have a trailer connected? Have you had a trailer connected lately? Have you installed anything on the truck lately such as trailer wiring, lights, etc?