I had this problem about a year ago when I started to do more high impact and step workouts. I think what I had was Achilles tendonitis. I used ice in the area, took a little break from the high impact and step workouts, wore flatter shoes, did some stretches for the area, and it eventually went away. I did have my doctor take a look at the area to make sure it wasn't anything more than tendonitis though.
HTH,
Michelle