Hostess Gift

I attended my bridal shower today that was given to me by my fiance`s mother`s friends. I was informed AFTER the shower by my mother-in-law (through my fiance`) that I had not bought a gift for the hostess and that she would cover me "this one time". She had already taken her flowers as we arrived. I was not aware of this tradition and perhaps it would have been polite for me to bring a gift. I am from a rural area and have never seen this done where the guest of honor is expected to bring a gift to her own shower. Is this expected or does it depend on the "area" in which you were raised? I would like to know if I am in the wrong and if I should send a letter of apology to the hostess. I feel horrible at my mistake and hurt by my new mother-in-law. If she had mentioned it before I would have gladly taken something. I feel as if I am beneath her standards. Please share your advice. Thank you.