<human>: This was my 3rd year of high school and had been dating a girl for a bout a year or two. We had a great relationship, or so I thought, at the time and nothing seemed out of the ordinary. One weekend I was hanging out with her and she went home that night and I called her to say good night (again nothing out of the ordinary). The next morning I woke up with literally the worst stomach ache I've ever had, like the anxiety ones before a big test or a sports game. I texted my girlfriend that morning and asked her "I have this weird feeling in my stomach, did you do anything last night after you left my house?" She of course responds, "No, I went to bed after we talked on the phone....". I brushed it off but I still had the worst stomach ache, so bad that I couldn't even my breakfast. I think it was no more than 2 days that her older sister called me on the phone telling me that she witnessed my girlfriend with another guy the same night she had come over (the night previous of the stomach ache). I confronted my girlfriend about why her own sister would accuse her of doing such a thing, which ended up in her confessing that she in fact did cheat on me that night. 

So reddit do you have any stories that your gut instinct was actually telling you the truth?

Can you summarize the given text in a few sentences?
<bot>: Woke up with a stomach ache sensing something was wrong with girlfriend, found out she in fact cheated on me the previous night.