What Causes Brain Fog And Pain Behind Head After Taking Pain Medication?

Hi, I began taking a generic form of hydrocodone for several pain related illnesses that I have, which includes body aches in lower back, knees, muscular pain in both arms and spasms in my right shoulder blade (most recently across the entire back), up the right side of my neck. Also suffer from Trimengial Neurolgia, Pituitary Gland Tumor and a Menogiomia . Last he suggested to my pharmacist that I take the brand form of the pain medication because I was still experiencing the brain fog and the some of the pain lingering somewhat in the back of my head. To my surprise, oh to surprise it hit every area of pain in my head (and I have several headaches sometimes going on at the same time), the brain fog was gone and no background pain lingering anymore. What s going on here?