I get that at night sometimes when it's right after I've gone to bed. What I do, as hard as it is I know! is to lay flat on your back and don't move a muscle. Concentrate on your legs first, and then in you mind tell yourself "now I'm relaxing my hips, then, belly, hands, arms, neck. Pretty soon that your whole body is relaxed completely. This is also a great way to get relaxed and fall asleep for insomnia. Please try this, it works for me!