Why is it that every time I do a major operation I start to shiver and feel very cold afterwards?

I’m not sure if you’re referring to doing anything major at all, or if you’re a surgeon and you have these strange feelings afterwards.  Either way you should visit a doctor.  If theses are symptoms that occur after strenuous physical activity you should visit your regular doctor, or go to a walk-in clinic if you don’t have a doctor.  You are indeed a surgeon then you should see both a regular doctor and a psychiatrist. 

Good luck with that.