Spasmodic pain in leg. Worry about it?

Sometimes, when I'm getting up in the morning from my bed, I feel I'm unable to move my one of the legs. And there is severe spasmodic pain in my leg. Appears as if my tibia will break if I try to move my leg even a little. At the same time the pain is so intense that I do feel the need of bending my legs so as to relieve myself from the pain. It lasts for few seconds, about 10 seconds or so, and then everything becomes normal, except that the sensation of bone-breaking pain lingers for further few minutes. Why does it happen? And do I need to worry about it? I've been ignoring it thus far. It is just now that I'm sharing it with anyone for the first time.