SUBREDDIT: r/AskReddit
TITLE: I found out my girlfriend is cheating on me, what do I do next?
POST: Me and My SO have been going out full time for 6 months, having know each other for about 18 months. Today, while she was in the bathroom, her phone went off, the guy who messaged her was one of her friends that I knew through her, and he was a pretty cool guy so I decided to text back for her (as I thought it was just pleasantries) and my gf is known to take hours in the bathroom.

Anyway, I open up the text thread, and the exact wording of the text was "I gt some doms you cool if i pick you up tmrw? ;) xx". I scroll further up and its the same inane sexual innuendo and even my gf is saying stuff like this. (I managed to read 4 months worth of texts.

Now, my gf told me she's going to see a movie with her sister tomorrow, but I asked her sister and she said they (my gf and her mother) were going out.

Everything is just adding up, what do I do?
TL;DR: 
Guy sends my gf a text implying their having sex tomorrow (as a recurring thing) and she's lied to me, what do I do?