I have been getting these since starting the diet too! Kept meaning to ask about it on here, so thanks for beating me to it.
Unfortunately I have no idea what causes them or what to do about it, but I’ll be following this thread with interest to see if anyone comes up with anything useful.
I did also have cramps like this during pregnancy, and they were much less of a problem if I did some yoga-style leg stretches before going to bed (as it was mainly only a problem at night). Anything that stretches the calf muscles is good.