I'm going to give my gf shit about her instagram
Why do you plan on giving your girlfriend grief about her Instagram?
Because I don't really like what she is posting on it.
Your gf's instagram isn't something to get worked up over, and insulting her over it is incorrect. Let the small things like this go.
I'm going to fix it by making her feel bad.
There is no reason to make her feel bad about her Instagram. She is free to do whatever he wants on social media if it's not harming you. I recommend focusing your mind elsewhere that way it doesn't affect you as much.