<human>: Can you provide a brief overview of the text?

I've been dating this girl for almost two months. It's the most serious relationship that I've been in and, as many lesbian relationships do, it has moved really fast, but I'm so happy with her. We have a great time together, we have great sex, we communicate, etc. I really like her. 

I only have one concern. She dated her last girlfriend for two years and they were very, very in love. They broke up only because the girlfriend went away to school, and even then they were hooking up for a few months when she was home from breaks. My gf told me all of this and it's fine with me because she was honest. However, she told me that everything between them was over. 

Yesterday she checked her Facebook in front of me and there was a new message from her ex. It said, "I love you, so I'll..." and that's all I saw because she immediately minimized the page and changed the subject. I should have asked about it then, but I didn't. And she acted like it never happened. 

It bothers me because 1) she said that everything was over between them and 2) she immediately hid it from me. If it were something that hadn't happened before, she would have probably read it, right? 

I guess I'm just feeling jealous and insecure and I want to know if this sounds like an important red flag.
<bot>: Girlfriend got message from ex that said, "I love you," and she hid it from me, should I worry?