Caught my gf cheating, need advice!!

I dont even know what to do. So basically I was meant to go for some special training as I'm a QB/WR rookie back up for a D1 team.It was meant to last a week but I ended up only going for 5 days. So I didn't tell my gf (f18) because I wanted to suprise her.(we've been together for about 17 months,so it was somewhat serious.

when I got back everything was normal. I opened the door. Went into the bedroom and barely had to open the door and saw the last thing I could ever imagine. she was in bed with another guy!. She saw me but I quickly just closed the door and left.I sent her a message basically saying "let me know when I can come get all my things".( it's been a few hours since the incident and (she's been blowing up my phone non stop).The thing that makes it a 100 times worse is that she's one of the main cheerleaders for our football team. So basically I'll see atleast 5 times a week (shits gonna be akward as fuck).I'm just confused "like is this some sort of crazy nightmare?". What do I do now?