I'm so sorry this happened, Cilleena, your outfit sounds absolutely beautiful and I hope you'll still wear it proudly in the future;-)
Like the other lasses have said already: there might be several reasons why she said this, but none of them makes it alright, it was rude and she made you feel bad. I'm also not very happy with the other members of your book club, they should have supported you (especially since they were wearing similar outfits)!
I absolutely understand that you don't want to stop hanging out with them and there are some good suggestions here as how to respond in the future.
I would just keep on turning up looking your usual fab self and, as strange as it may sound now, I wouldn't be surprised if you started getting admiring looks and positive comments in the future.