Rashes are usually an allergic reaction to something. Have you changed your soap, laundry detergent or softener? Have you eaten any new or different foods lately? Are you taking any meds that you have not taken before (including herbal and over the counter stuff)? If you are on pain meds that you haven't taken before, you need to stop taking them & call the doc who ordered them, today. Allergic reactions to meds can be life threatening. Best of luck with your surgery, and I hope your rash gets better!
Did the rash start when you started taking the pain meds? You may be having an allergic reaction to the medicine. Maybe try some benedryl or some cortisone cream to see if that helps? Keep us posted!
Kim