I think I am going to post a snapchat story about my girlfriend using her account.
Why are you going to post a story from her account?
I plan on posting some of her secrets in order to hurt her.
You shouldn't share someone's secrets. That could hurt and embarrass them and they will be angry with you when they found out you told them.
I just want to make fun of her anyways.
Be careful not to expose anything that your girlfriend may have wanted to keep private. Going on her social media just to talk about her secrets is pretty low, and if she finds out then odds are you is going to be very upset with you. This could even cause you two to separate.