Hi & Welcome. incomplete data like age, gender, purpose of taking Tegretol?? Based on the history given in your query, you seem to have developed hypokalemia (fall in serum potassium level) due to excessive potassium loss while having great bowel motions. You need to undergo certain lab tests such as serum potassium level, & ECG. This condition is reversible if the serum potassium level is restored to its normal value by given potassium supplements or IV potassium depending upon the severity. One of the important side effects of Oxycodone is constipation which usually does not occur so early. Tegretol enhances the clearance of Oxycodone from the body, thus reducing the blood levels of Oxycodone thereby reducing its efficacy in treating your post operative pain. You may need an alternative analgesic to overcome this interaction. Hope this answers your query.