I am not pre op yet, have only just started the process. The headache day 3-4 is withdrawal from carbs/sugar and you likely feel run down and tired also?? I was on a high protein and extreme low carb diet before and lost 60 pounds, but have gained most of it all back now I got the headache day 3 and it lasted a couple days, I was told that it was fro withdrawal from carbs/sugar. It will go away. Also you will likely have issues with being cold pretty soon. I was always in a sweater, even in summer.