Why would a guy go from being normal around a girl to being nervous?

I've known him for about a year we work together. And in the beginning he talked to me without a problem. Now after one day we locked eyes with each other things have been weird. When he talks his voice shake's and it seems like he can barely breathe or get his words out straight his cheeks slightly turn red. He's a pretty confident guy. I noticed when he talks to other females at work he sounds nothing like that, he sounds clear and comfortable. And he's the one majority of the time trying to initiate conversation with me.

Why would a guy go from being normal around a girl to being nervous?
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