It's normal. I didn't get the flu symptoms when I started but I did have pretty bad constipation, and headaches. They went away like a light switch after 7 days. I'm sure everyone's a little different.
Just remember, keep an eye on calories too. Keto is an excellent way to restrict calories while still getting sufficient nutrients, but it's still possible to overeat on healthy fats. Especially if you love nut butters. Fucking nut butters.