Sounds like you need to have a heart-to-heart talk with her, and explain what you just told us: that you are honored to be her matron of honor, and will certainly be able to be at the wedding and do all of that, but that you won't be able to leave your baby for a bachelorette party, and are not sure if you can swing organizing a bridal shower. That you feel that these things go with being the matron of honor, and that if she is expecting that as well, that you won't be able to do that, and you are fine with stepping down from the post since you can't "fulfill the duties" that go with it. However, if she would like another of her friends to step up and do that w/o being a matron (or maid) of honor, then you will do that.
For my part, I never knew that the maid/matron of honor was supposed to do anything, except be the one standing closest to the bride at the wedding (but then, I'm not tremendously up on wedding etiquette). My MOH didn't do that for me, but then, she lived several hours away, so couldn't organize a shower.
If she's a real friend, she will understand and graciously let you off the hook of the duties with or without naming someone else MOH. If she can't be gracious, then that may be an indication that she's not quite as good a friend as you think. :-/