Girlfriend texts guy behind back?

The reason why I don't like it when my girlfriend texts other guys is because the last time she texted a guy he was flirting with her and she went with it. She didn't stop him.

Now she promises not to text other guys.

I then realize I was a bit controlling because she didn't want to upset me by texting other guys.

I told her than and I said that as long as she is 100% honest with me and who she texts, then she could text anyone.

I asked who else she's been texting but me and it was one of my friends. I knew they texted before because she told me she only texted him once to ask him a question about school. She said they have been texting a lot more now. About every other day or so. She didn't tell me they were texting until now.

Why did she text that guy when she knows it would upset me? She even told me she wouldn't because she knows that would harm me. She never told me she was texting someone else until I asked her.