I've been having the exact same issue...only mine is affecting my right calf. But yeah, feels like it's in a constant state of contraction all the time. Mine started during a run about 3 weeks ago, and has been intermittent ever since. Somedays it's horrible and hurts just walking...other days it's not bad at all and I barely notice it. I've been foam rolling it, which helps a little bit. And I've been taking Cal/Mag/Vit. D supplements for years, so I've got that covered. But I noticed the biggest improvement when I bought a compression calf sleeve at my LRS and started wearing it during my runs and immediately after. I've even slept with it on a few nights and in the morning, my calf feels fresh and ready to go. I haven't had a single "seize up" during a run or ride since I started wearing it. The store I went to had 2 different brands - Zensah and CW-X. I ended up going with the CW-X one, only because I already own a few other CW-X items. They come in a pair, and were about $40. Might be something worth considering.