Am I stupid?

My girlfriend and I have been together for almost 3 months and been living together since before that. This is my first relationship with a female, and everything seems great. But i got a text from someone i used to consider my best friend that my woman has been cheating on me with everyone she can since we got together. I don't and didn't believe her. My girlfriend and her used to be friends, and my 'used to be friend' had a massive crush on her. The so called friend, had been trying to break us up since we got together. My girlfriend has a history of being a cheater, but she's serious when she looks new dead in the eye and says she has never cheated on me. I believe her, but I've never been the one to believe anyone once that's called into play. So, even though I'm inclined to believe my girlfriend... i worry that i might be blinded and denying something that would be important to know. I should just let it go, but I'm curious as to if I'm overlooking something I shouldn't. Like, am i overlooking it because i don't want to believe it or because it's not worth it? I don't believe it because i don't feel like my woman would do that. Not because i think I'm all that, but because i just don't feel it. My intuition hasn't steered me wrong thus far. But everything is so different with her than with anyone I've ever been with. So, I wonder if my believing her instead of the exfriend is an emotional override or if I really don't have to worry about it.