Why does my ex boyfriend's face get red when he sees me??

well I have recently been transfered to my ex's department at work

and he is sort of like a supervisor and when he has to get close to my desk to deliver something to somebody else he blushes. His face turns red completely and he nervously smiles hahaha

He broke up with me like 6 months; we haven't spoke since then
Why does my ex boyfriend's face get red when he sees me??
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