nightly jerking

Im having pain-mangement for my back, take hydrocodone and morphene. Im not a complete zombie and have a very happy married life with a three year old. Last couple of weeks though ive been waking up regularly in the night with some sort of jerking, only way i can explain it. I guess it comes from my legs but not sure. Its like when you dream and your falling and then suddenly awake, well its that sort of feeling. My meds havent changed, if anything i try and take bit less and less each day so basically i havent done anything drastic or sudden recently and no stresses apart from waking in mid of night kinda scared. Anyone got any ideas?? Would really appreciate it.