Can you help me understand why talking about underwear seems to make some people uncomfortable?

Today I went to the mall with one of my closest friends and her boyfriend. We went into one of the shops looking for nothing in particular when we saw that they had some really cute underwear on sale. She asked me if I thought it was a good deal and how much is too much to spend on undies. I told her I wasn’t sure how much they usually cost here in the States because I’ve always bought mine overseas, so I couldn’t give her an honest opinion as to whether they were a good deal or not. She got really flustered and said “I don’t want to talk about this anymore” and proceeded to put the items down and walk away from that section of the store. I’ve been really confused all day about her reaction. I don’t think I said anything wrong and her boyfriend wasn’t even within earshot (he was over at the men’s section) which, if he were, would’ve been the only plausible explanation in my mind as to why she could’ve gotten so embarrassed. And what’s even more confusing is that she’s one of my closest friends and we’ve talked about raunchier things in the past and I’ve even helped her fold her laundry, undergarments and all…and we’re both grown women (24).

From your own experience, do you find that talking about underwear makes people uncomfortable? Why do you think that is? Also, can you please help me figure out what, in the scenario I presented, could’ve caused my friend to get so flustered? Maybe I’m just not seeing it?