Why did my guy friend act so strange when he saw me in my sports bra?

Me and my guy friend are roommates and we are both straight. He knocks on my door to just hang in my room. But I had on a sports bra and an oversized button up shirt. He demanded I button up my shirt. I had this outfit on all day and it really wasn’t that big a deal. He told be before that he wasn’t attracted to me so I don’t understand the problem.

Why did my guy friend act so strange when he saw me in my sports bra?
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