What it means: The sudden stab of a foot cramp -- basically, the hard contraction of a muscle -- can be triggered by fleeting circumstances such as exercise or dehydration. But if it happens often, your diet may lack sufficient calcium, potassium, or magnesium. Pregnant women in the third trimester are especially vulnerable thanks to increased blood volume and reduced circulation to the feet.
More clues: Charley horses tend to rear up out of nowhere, often while you're just lying there. They can be a single sharp muscle spasm or come in waves. Either way, soreness can linger long afterward.
What to do: Try to flex the foot and massage the painful area. You may also be able to relax the muscle by applying a cold pack or rubbing alcohol. To prevent cramps, stretch your feet before you go to bed. Then drink a glass of warm milk (for the calcium).