Why do you get charlie horse cramps in leg after having back surgery?

Exercise calves: Most likely because the strain and discomfort of your back you have not exercised your calves so the accumulation of Lactic Acid in the calves causes cramps. Also rule out other causes like low potassium, low calcium or magnesium. Try tonic water, exercises doing ( standing on your toes) tip-toe. Some medicines could cause it ( cholesterol pills).

Acid buildup: While under anesthesia, the legs are bent and there is a decrease in blood return from the legs. Waste products like Lactic Acid build up. There is a decrease in blood supply and oxygen levels are lower, compounding the problem. After surgery, the legs are straightened, and blood and lymph fluids begin moving. Muscle cramping occurs during this time, when the local pH is low.