Why would a guy Go from super happy and chummy with a Girl to very Short and Cold within Minutes?

My coworker seemed very happy to See me when He arrived Today. However 10 Minutes Later when I asked him a Quick question He frowned and seemed like He Wants to get rid of me. I can't remember Doing anything to Provoke this Mood switch. Do you have any ideas?

Why would a guy Go from super happy and chummy with a Girl to very Short and Cold within Minutes?
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