Why does she do this?

So I have this friend I guess and she always gives me back handed compliments like it's annoying and I always respond nicely back. Like should I call her out on it cause I have once and she didn't know what to say and made a joke about it trying to brush it off. So like today I bought these really cute well to me they're cute underwear with the days of the week on them and I thought they were cute so I showed her them and she's like" they look like little kids underwear" which is her opinion but I didn't ask her for it she could've just said oh yea or lied to me about it but this isn't the first time I try to tell her something and she says something rude back like the other day I said I wouldn't wear this top that you have to have Confidence to wear it and she said yea cause you don't have any. And it got really quite after she said tht but I didn't say anything on purpose so she could know she went too far and that I heard her and then she changed the subject so fast . Like I don't get why she needs to bring me down to make her feel better.