Unsure why he got mad at this.?

So the other day one of my best friends (a guy) and I were hanging out. We were chilling in my room, as always, watching T.V casually before going to see a movie with some other friends. I noticed we had to leave in half an hour so I got up and changed my shirt. Then I looked in the mirror, decided I didn't like it, and took it off and put on a different one. Then I slipped off my sweatpants and put on some jeans. As I was doing up my pants he suddenly said, "You know, you really shouldn't do that," then sat in silence looking pissed off. I kind of looked at him quizzically, then continued putting on accersories and making sure I had everything. My question is, what the hell is he all mad about? I don't think it's a big deal so change like that infront of friends, you walk around in a bikini, what's so wrong with a flash of a bra or underwear when I change a shirt or pants?