Mom has dementia. How do I answer her questions?

My mother is 73. She has dementia, not severe, but it's there, and is changing her daily behavors. I recently change her from regular underwear to the pull up disposable underwear for women, for about 3 weeks now. Everyday, many times a day, she keeps asking me for her underwear. The reason she has to wear this new disposable underwear is because when she was wearing the regular one, she keep changing it a few times a day. I observed her and found out why she did that: she did not clean herself throughly after each bathroom visit (she has pee stain or even dirty from #2) so she changed and washed the dirty underwears (at least 3 per day). I did tell her that the kind of underwear you're wearing now is good from bath to bath (change it when you take a shower daily - she still goes to the bathroom herself and normal, I now also make sure she is cleaned) so there's no need to change it few times a day.
I do not want to ignore her or give her a cold answer "no, we dont have it!" because it will make her feel ignored. But at the same time what else can I reply to her to make her feel better? I can't explain things to her because she won't get it anymore. I feel so sad for my mom.