I go with what Kelsey said about the potassium. When pregnant, one has to be alert to the risk of deep vein thrombosis, which can be signalled by severe leg pain. But DVT doesn't ease when you lie down, and it doesn't happen in both legs at the same time (you'd have to have a clot in both legs that happen coincidentally, which would be almost impossible). It does sound like a muscle protest of some kind, try the potassium and possibly some magnesium or phosphorus.
Maybe your potassium levels are low I know that muscle soreness in the legs is common in pregnancy because of potassium. Try eating a banana or two to get your levels up. And stretching your legs. If that doesn't work maybe just Tylenol.