Why is my "best friend" doing this to me?

I dated this guy and she knew. He knew we were cool. She then proceeds to follow him on IG and he did too. Okay, not really a big deal. She then proceeds to send a picture of him following her back. didn't really think much of it. the next day she proceeds to text me saying "she thinks that he likes her" I didn't really think much of it at that time. Today was her birthday and she sent me a screenshot with him saying happy birthday to her. I got annoyed and I blocked her. Should I cut her off and why does she do this if he's my ex?
Why is my "best friend" doing this to me?
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