Why would my ex completely ignore me in person when I say hi in passing?

We both go to the same church and we sometimes run into each other. Three times I have tried to say hello and she just flat out ignored me. One when I passed her in the halls. She did not even move her head to look. Once I was playing pool and she came to put her stuff down. I said hi and she ignored me sat down for a few minutes not 6 feet from me. She turned around looked around at the social groups for 2 minutes and walked off. The last time she was talking with my friends. I came over not having seen her. I said hi to every one she ignored me and a few seconds latter broke off the social group with one of my friends yo make thier own social group.

I get blocking me on social media but why actively ignore me in person?
Why would my ex completely ignore me in person when I say hi in passing?
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