What Causes Involuntary Jerks In Arms And Legs?

The past two nights I have suddenly developed severe arm and leg jerking.....like I'm having a grand mal seizure but awake and aware of the jerking. The jerking is so severe it wakes my husband and he thought about taking me to an ER. If I try hard, I can stop the movements for a short time, but when my control wears out after a few seconds, the movements are even more severe. The movements last 1-2 hours. What is their cause and how do I stop them?