I was hoping you wouldn’t say that. How painful is it? My daughter got on her tip toes to reach for something, and snapped her tendon. I hope that’s not about to happen to you. It’s very painful. Only her problem was the tendon in the back of her calf.
Off the top of my head, I’m wondering how your feet hit the ground when you walk. If you have a gait problem, it could cause what you are describing. You might want to try some inserts in your shoes. Spenco makes arch supports that you can find in almost any drugstore. They cost about $30.00. Give them a try, and if they help, you might want to see a podiatrist and get some orthotics made. they are custom molded to your foot, and they slip in your shoe as an insert. They correct a lot of gait disturbances, and could alleviate the problem you are describing.
Here’s another link that might be more helpful: http://www.sportsinjuryclinic.net/cybertherapist/front/lowerleg/inflamtibtensheath.htm