It upsets me that my girlfriend won't let me come visit her at work because she wants to flirt with her coworkers.
Why does she want to flirt with guys at her work?
I think she wants to get laid.
It doesn't make any sense for you to be in a relationship if your girlfriend wants to flirt and get laid by other guys and you're not comfortable with this. You should speak with her about this and leave her if you can't both agree on boundaries.
I'm being unreasonable and I can't understand anything.
You are not being unreasonable. If you are not in an open relationship, it's perfectly naturally to be upset with her choosing to flirt with people instead of letting you visit her at work. You need to talk with her about this so she knows how upset this is making you.