Caught my ex boyfriend staring at me?

So I saw my ex boyfriend for the first time in almost a year, whenever he first saw me I caught him staring at me for at least 10 straight seconds and then after that it was nothing. He would not even look my way or acknowledge me and he started being really flirty with other girls at work after that in front of me. He slapped one of my friends asses right in front of me and he used to never do that kind of stuff. Then he took off his shirt and threw it in front of everyone acting kind of full of himself what is the meaning behind this?
Caught my ex boyfriend staring at me?
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