I also got Phenergan, for something (not post-surgery...everything they gave me for nausea then was injected). But I wonder if maybe it's not the oxycontin, but your digestive system "waking up" from the anesthesia. It can take several days to a week for it to work properly again.

Hope you're doing better now that you've been able to eat a little bit. Try and get fiber to counter those pain meds...