What Causes Hot Flashes While On Narcotic Pain Medicine After Knee Replacement?

I had a total knee replacement 11/17/14. I have been off all narcotic pain meds since 1/1/15, and only take Mobic (an NSAID) at night. I have been experiencing hot flashes throughout the day and night ever since going off of the pain meds over 3 weeks ago. I had controlled hot flashes from perimenopause prior to surgery by taking Evening Primrose Oil, and I take the same dose now, but it does not seem to help anymore. Are the hot flashes from the surgery? Or from the meds? Or has it triggered a menopausal symptom?