Maybe she's trying to heal the relationship? Maybe she was in a bad place and couldn't function/didn't want to bring you down?
But why is she planning her own shower? That's not kosher at all. If you think she's just being selfish, you have a problem.
You've agreed to be matron of honor so you kind of have to do the shower with the maid of honor, but I don't know why the bride is planning it.
If you sincerely want to be there for her, then be there, with all your heart. If she starts racking up bills for you, you can say, "I know you can relate to wanting to keep expenses reasonable and I know you will be understanding." If you get any kind of flak from her, then recognize that she has already given you an "out" and that you aren't really such great friends anymore.
Your call. Just ask yourself how much you can depend on this relationship, and how much you will miss it if it's gone.