See a doctor. My concern would be a vascular problem with a possible blood clot.
Muscle /bone imbalan. 1. Check to see if you have one leg shorter than the other? 2. Check to see if you have lack of ankle joint dorsiflexion. 3. Are you over pronating (flat feet) in stance phase of gait? 4. Are you doing passive muscle stretching exercises prior to walking? 5. May need some type of orthotic. 6. Myofascial trigger point release of the posterior muscle group will help.