My girlfriend says she doesn't love me anymore, is it true?

Sorry to hear this. But sometimes the urge of thinking what if is toooo strong. She'll probably regret it, and does love you, but thinks life is too short for what ifs!

the very same thing happened to me, exact same lines and circumstances, but here's what I did. I refused to leave without a proper explanation wondering why me who cares about this girl with all of my being and some guy that's done jack for her is all of a sudden more important than me. I told her I loved her and she didn't say it back. I probably sound like a mug doing it but I knew that something was up at the time. I knew she flirted with him by text and saw every saturday night as a flirting session but I stayed still. After about 2 months of this and countless talks, we decided that she was going to give me a "chance" even though I did nothing wrong in the first place. After that I completely submitted my life to her and really made her see that I care. After about a month of this and after everything was back to normal I told her that I didn't want to be with her anymore and her reaction was amazing. I realise it might sound horrible to play with someones emotions like that but I wanted her to see how much she means to me and what losing me would mean for her. She cried collapsed to the floor begged and pleaded for me to stay, so I did. I told her that if at any point she wanted to break up with me that she could and that I would never break up with her.

Sorry if it doesn't make sense I'm just typing while pulling from my memory banks.

The point in all that is that although you would never beleive it girls can fall out of love temporarily as a sort of "phase". You just need to show her what you can give to her that nobody else can.
I am still with the same girl after 5 years.

To just a random guy: telling her how he feels is getting emotional but otherwise on the ball.

It's happened to the best of us. You better let this one go and try to have some fun in your life without her. She's already strayed emotionally from you and it can only get worse from here. Sorry for your loss, but that's life. We can't force people to love us. If you want to leave a lasting impression on her, talk to her in a calm tone and, whatever you do, DO NOT GET EMOTIONAL - they feed off that stuff. Tell her how you feel and leave it at that. She'll probably say that things aren't the same but she still loves you, but you already know it's coming. If you're both mature enough you can stay friends. And then, maybe one day, she'll regret what she did and come back to you. But you won't care by then because you already have several girls interested in you that don't hae a history with you.