My guess would be that she already knows you've been rating her panty drawer and wearing hers and that's why she bought you your own to keep you from stretching out the waistbands of hers
Maybe I'll just have her sit on it and then have her play with herself until she has enough orgasms that her contractions finally set me off inside of her
If she bought them specifically for you I rather doubt if she's going to be that surprised when you wear them as why else would she buy them for you if she doesn't want to see you in them