Why is my ex being such a bitch?!

So I broke up with her last month and I thought we were cool. I told her we weren't right for each other and that we were too different to last, she agreed and said we could still be friends. But I just found out today that she's been talking sh*t behind my back. The worst has to be that she says that my d*** is small and that I suck in bed. Wtf right?! I was always a good lover and my d*** isn't small, its a good 6.5 inches, she was there when I measured it. So could anyone shed some light on my situation? Should I confront her or just ignore it?
Why is my ex being such a bitch?!
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