That doesn't sound like withdrawl. Although even if your still taking narcotic pain meds you would still have moriphine withdrawl, your body is use to that one so when not taking it your body should withdrawl.
Anyway, it sounds like the Flu that's going threw our office right now. Same exact symptoms so maybe you have a Flu. The constipation is one that's not on the Flu list but since you're still taking pain meds this is probably causing that.
Could always go to your PCP and get tested to see what you have.