Is my girlfriend cheating on me with my old good friend?

I am 17 years old and have been going out with my girlfriend on and off for about 2 years. She just came to my school this year. Day 1 of this school year my old friend stopped talking to me, he started ignoring my calls and ignoring my texts. Oddly enough, this guy was the one who has always told me to break up with my girlfriend. As the year continued, my girlfriend and my old friend got extremely close to each other and started to frequently go off campus with each other… alone. Whenever i tell them that i will go with them my old friend backs off and doesn’t go. I always see them laughing and flirting with each other. When i ask my girlfriend WTF is up, she just charms me (aka has sex with me) and then says he’s just her only good good friend at school. she also says "we just talk about his girlfriend" but he first of all hasn’t been with this girl for the past 5 months. What should i think? What should i do?

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