Opioid analgesic sweating, pathophysiology

Hi all!

So my question is this. I take care of post ortho-surgery patients generally for 3 days before they go home and I've found that Percocet 3/325 in varying amounts works for most people. I've run across the symptom of diaphoresis within an hour of administration though with enough of my patients that I feel pretty confident in linking the two incidences. Patients have asked me why this is happening and I'm drawing a blank. I've tried to research this on my own before asking a stupid question but have, so far, found no answer.

Is this just idiopathic? (which i hate that diagnosis, if somethings happening with my patient I want to know what and why!) These patients are generally healthy adults who have had hip or knee surgeries with stable vitals. Sometimes the vitals are a little high or a little low but for the most part these patients are stable. So i'm left to wonder how this medication is causing this reaction.

Any pointers or help would be greatly appreciated!!!