This can be due to over stress or overuse of muscles during long standing /running/any wrong twisting movements. Consult a physiotherapist..
Next Steps
Go to a physiotherapist.. TENS +some Heating therapy +some strengthening as well as stretching exercises will be helpful for sure.
Health Tips
At home.. Rest +hot water fomentation +pain killer gel +ankle - toe movements +no leg hanging +no cold exposure +no long standing /walking /travelling +no stair climbing.