What would cause my 3 year old to jerk when falling asleep?

My daughter has been sick for 3 days with a bad cough, and will not eat or drink. We went to the ER today and they determined after chest x-rays that she has pneumonia. She has not been getting good sleep from coughing all night. My question is, why does her body jerk (like when you fall in your dreams) right as she's falling asleep? She does it probably 10 times, and once she's asleep the jerking stops. Could it be from extreme fatigue or dehydration? She's never done this before. Thanx for any help!