What Causes Discomfort In Legs After Waking Up?

This has happened three times these past two weeks. I wake up and am able to swing my legs off the bed to get up (as I normally do - feeling normal), but when I get up my legs are like jello and don't hold me up. I can't even feel my legs. I fell twice because it is so unexpected. It goes away after a few minutes. Should I be worried?