<human>: A little history.
We've been dating on and off for the last 3 years and till a few weeks ago I thought we really had something. Now here's the story.

So we're both high school seniors and yesterday she was on her facebook on my tablet during class and it was a usual day but today I opened up my tablet and realized she was still logged on. 

There was a notification saying someone responding to her inbox so being the inquisitive person I am I decided to check the inbox and it was reply from some guy, normally I wouldn't have gave it a second thought but I was a semi-colon parenthesis and decided to read their conversation. 

As I read on I picked up a few things, he worked at the same place as her, he was flirting with her.. and she was flirting back

This had been going on for the last 4 weeks during which I've noticed she talked with me a lot less and was unresponsive most of the time.

Then I saw a second inbox with her talking with a friend that i did know, I read a message from him asking if she was still single and she replied saying that she was seeing someone, I was temporarily relieved but when I read on she mentioned that he was a co-worker..

At this point I don't know what to do since she's not open about relationships and I don't know how to approach her, if I told her I was going through her messages she'll be really pissed but I don't know what else to say :/

Please help reddit

Can you summarize the given text in a few sentences?
<bot>: Girlfriend pretty much dumped me without informing me yet still holds my hand and etc while this whole time she's been seeing someone else.