I (18M) have been dating my girlfriend (18F) for six months now.

Recently, I've felt our relationship kind of decline. Lower sex drive on her part, doesn't want to just sit there and kiss. All I've been getting these last two weeks is just her sitting on my lap, cuddling.

So I was driving her around to get ice cream about 3 days ago, and it was like 9PM, dark enough to where I can obviously tell she's texting, (bright ass screen), I look over and ask her who Doug is.

Her : She's one of my brothers friends, they graduated together. Me : Why are you talking to him? Her : I'm going to go to his birthday party in a week or two Me : Since when? Her : I haven't told you because I'm still trying to figure out who I'm going with, right now Sosa is going with me. (one of her lady friends) Me : ... Her : I would have asked you but I didn't want her to be a third wheel.

Now, this conversation raised some red flags for me. This is my first LTR and you know, I love her, blah blah blah. Introduce another guy, with sketchy information doesn't really help.

So I talked to her that night, was like hey I don't want him flirting with you, or anything like that. Make sure he knows you have a boyfriend?

her: He does, we've established a long time ago we didn't want to be anything more then friends.

Anyways, I was texting her last night around 11PM.

her:im going to bed, goodnight me : Okay. I love you her : i love you too

Then I go to my friends house, chilling out playing Xbox and I get a message from her at 1:30AM

her:I went to dougs me:Why her:I was bored. me:When did you go? her:im on my way now me:Who are you going with? her:with doug (she doesn't drive, so this asshat drove all the way down there to pick her up) me:I assume you're staying the night? her:yeah me:have a good one. her:I'm sorry. Me:I just want to let you know, it's not okay and we'll talk about this the next time I see you. Are you going to be home tomorrow? her:I know. Yeah.

So then this morning, she takes down our relationship status on FB, changes her profile pic from us, to her and her family.

Anyways, she's home now. Waiting for me to come over.

I don't know what the hell to do, I've browsed some of TRP and know I shouldn't be lenient with her.

Pretty much not even sure I want to be with her after this.

So, when we break up, she'll want to be friends. What should I do?

Help appreciated. Thanks TRP.

TL;DR, Girlfriend goes behind my back to stay the night at some guys house. Haven't talked to her about the details yet, so I don't know what went down. She took our relationship status down. What should I do?