What Causes Muscle Cramps In The Legs While On Morphine?

I am a 60-year-old male Caucasian in generally good health. I suffer from chronic pain in my neck and take morphine and hydrocodone daily. Recently I have been experiencing severe muscle cramps in the Semitendinosus muscles in the back of my legs. They are so strong and painful it causes me to collapse on the floor and my pain level shoots up close to a 10. Can dehydration cause muscle cramps. I take miralax every day. I am wondering if it might be sucking water out of my blood causing my muscles to cramp. I poked my finger in the blood that came out seemed thicker than it normally is. I cook all my own food so I think it s possible I might not be getting enough salt. Any suggestions?