BFF gf

(AITAH) my BFF of 3 years started dating this bish who called her a slut in front of me without knowing I was her BFF. When this gf let's name her sam. When sam met me with my BFF she turned blue because she was talking to me about my BFF and I had no clue they were together. Now when my BFF (Sarah) came over I told her and she said we didn't have the same class so how would I know if she said that. I said ok sorry and we went on with our day. I believe Sarah didn't want to believe me because she has had her heart broken to many times and she knows I never lie to her about that. And is it bad I want to break up the relationship?

Let me know