Is it possible she only invited you to be a bridesmaid because the groom wants your husband to stand up with him?
I think you should bow out anyway. You don't like the bride, how can you support her? This is what I think of when people say someone is being P/A. You said yes to something you didn't want to do, now you regret it but won't stand up and say forget it. Instead you'll be sort of nice to her face and talk smack behind her back . That's kind of cruel to the bride, no matter how you feel about her. Do it now before you spend any money, and give her a chance to ask someone else.
I don't know where the assumption came that I am talking "smack" behind her back. I simply posted about it here..I'm not trotting around airing dirty laundry all over town 
Perhaps smack is a little strong, but your feelings are very clear. And just because you don't see us, we're still people to whom you are gossiping about this woman.
This is not someone you consider a friend. You accepted the "honour" of being her bridesmaid when you really didn't want to. You "immediatly told her that anything I could do to help with the planning process would be my pleasure" and yet when she actually asks for something you get angry, "I mean very angry."
Whoa. That's not okay.
But let's pretend you are a good friend of hers and are here for a sanity check.
It sounds like it's 10 weeks from her wedding and she's panicing a little. You are the last BM dress hold out. You have given two different reasons (size and money) for not getting your dress yet. You've known since December what your dress would cost, yet you have nothing set aside for it. I'll trust that you have succeeded in your mission to get into that smaller post-baby size. It's not unrealistic to have your BM dress a couple months before hand. It's great you can order a bridesmaid dress that quickly, most places recommend at least 6 weeks. Maybe she's nervous her deadlines are unrealistic. A couple of weeks isn't much time to fix any problems. Since you are married, you know it takes much less time for a tux (they just pull the correct size from the warehouse) than a bridesmaid dress requiring alterations. It's super lucky your GMIL can alter your dress that quickly as well, most have to fit into a dress shop's schedule, and after the alteration nightmare my friend recently went through, it's totally understandable not waiting until the last minute.
You seem to be avoiding communicating with the bride who's wedding you are supposed to be helping organize. Why is your DH talking to her DF instead of you two talking to each other?
She's okay, maybe a little stressed, why are you so angry?
It's clear that the Groom is a dear friend, so for his sake, either bow out before any money (and more hard feelings) are spent, or suck it up, think positive thoughts and try to keep your feelings to yourself. It would be terrible if your behaviour and attitude was the end of the rel
ationship between your DH and his friend.
It would be worse if she actually likes you, and considers you close enough to stand up with her, and this is how she's rewarded for her esteem. That's why I suggested maybe she feels the same way and only asked you because she had to, for your DH's sake, and so he wouldn't be paired with another woman at the wedding while you sat and watched.