I am a Sr. research asst. in a research facility and also an rn. I worked with investigators and they have found that morphine injected in normal mice can show inflammation in the spinal cord histologically. So it is believed that even though morhpine alleviates pain it also may cause pain due to its inflammatory nature. I'm not saying that this is the reason, it may just be a sensitivity issue and you can no longer tolerate the drug, but there are studies out there showing the effects of this drug that is meant to alleviate pain.