Could you give me some advice?

I've been with my girlfriend, for 2 years now we're now 18. I love her and she loves me but i can't help the feeling she wants to be with someone else but whats really getting to me now is that her friend kacey has recently been cheated on, and i suggested my close friend, ( which i regret.). and lately my girlfriend has only been speaking about them. it annoys me because its like she's not even interested in us anymore. i caught her hiding something and she was laughing texting her friend telling me its 'girl talk'. she made a joke about him being 'hot' to me and now i can't kick the feeling she'd rather be with him. I do not want kacey and my friend to be together and i don't know why. What should i do.