If you're moving away from high heels to flats, do so gradually, wearing lower heels at first. That will give your Achilles' tendon time to relax and stretch out. You might also try doing some of the standard exercises for stretching the Achilles' tendon.
The basic maneuver involves keeping your heel on the ground and your leg straight as you lean forward.
Find out more about this book:Harvard Medical School Foot Care Basics: Preventing and treating common foot conditions