Dene may be right - you might be holding too much tension in your ankle and heel striking. Also, if you are over-striding, landing way out in front of you hips and hyperextending (locking) your knee, that can cause that issue. Keep in mind that you want to land behind your hips and never ever step in front of your knee.
One last thought - if you are leaning too much, you’ll feel tension in your calves (see page 234 in the Chi Running book). It doesn’t sound to me like that is the issue, but bear it mind.
See if that helps and keep us posted.
Certified Chi Running and Chi Walking Instructor