i've had these loads, both in pregnancy and not - so painful that I spend the day after limping - ended up in hospital at 21weeks with suspected DVT (but it wasnt) because of a leg cramp....
Anyway, I am not convinced anything reallys works in pregnancy (sorry) - been recommended Tonic water (for quinine), bananas (for potassium), bit of salt, calf stretches during the day etc - all worth a try in case it works for you.
When I had the worst one (when ended up in hosp), my H spent the next few days massaging my calf which really seemed to help.
I do sympathise x