Pain is a stress and can certainly result in high blood sugars. Also, injury (like surgery) results in inflammations that again can result in high blood sugars. Acetaminophen can interfere with blood sugar tests, but it usually causes false low readings and it is not a strong effect. I've not heard of codeine affecting blood sugars or test results. You could try Ibuprofen and see if there is a change, but it may just be that your body has reacted to the surgery with high blood sugars.