Have you asked her why she insisted on you carrying her bag? If you feel uncomfortable, maybe you should let her know your feeling. Most likely, she may have heard it somewhere that a really loyal bf won't mind doing anything for his lover, including carrying bags.
In the event that you can't bear it, maybe you can tease her when she requests you to carry her bag by saying that
"I don't mind carrying bags for you. Nonetheless, the bag (whatever brand it is, LV or Prada) that you are carrying is of such a bad taste that I am ashamed to carry it!" Of course, you have to prepared that your gf may ask you to buy her a bag that is of good taste!