Best friend got with my ex...

So I recently dumped my ex who cheated on me (found messages on his phone) and I vented to my best friend about it and this bitch actually is trying to tell me I'm going crazy because "he's not like that". And so she didn't believe me, I was like whatever I know it's true whether you believe me or not. Well she started dating him like 3 weeks later. After I told her he cheated on me. And I've been best friends with her for 5 years. Am I being over dramatic or??