Menu Header partially scrolls with page

Hello, I am very new to anything that has to do with coding. Recently my menu/header partially scrolls down with my page, and looks more like a glitch. I believe it started after trying to optimize the webpage for gtmetrix. Can anyone help me figure out how to keep the menu/header in place at the top of the page?

website: theafterlash.com

Please let me know what I need to post in order to help pinpoint the problem.
Thank you!