I posted a reply, but my computer ate it :-(
Are you sure that she now intends you to be a bridesmaid?
If so, I would speak to her (directly if you can, bit by e-mail if you have to) Explain that it is a little awkward, that it seems there has been a mis-communication, as until she sent the mail about the dresses you had not realised that she was considering you as a possible bridesmaid. Explain that unfortunately, due to your health and other family and personal issues, you are not in a position to be part of her wedding party.
I think the tone to take is to be clear that this is about *you* being *unable* to participate (if pushed, don't go into details, reiterate that it isn't possible, and if you have to, say it is for reasons of health (which it is; you don't need stress related illnesses on top of a pregnancy and recent surgery, and prevention is better than cure!)
The money she gave/lent you is a completely separate issue.
I hope that she won't try to link the two or to try to pressure you into being a bridesmaid after you have turned her down. If she does, it might work to appeal to her generosity ("We are so grateful for the help you gave us when we needed it. We really appreciated your support, and I know that you are far too generous and considerate a person to try to talk me into being in your wedding party now that I have explained that it won't be possible. I really appreciate your understanding that this is not something I can do" This makes it harder for her to turn round and say that actually, she isn't considerate or understanding enough to accept your decline, and gives her a graceful way out - if she likes the idea of being seen as generou and helping you out, you've given her a way to see herself in the same light by accepting your refusal.
When is the wedding? A "How kind of you to ask me, but I can't possibly accept - I would not want to take the risk that I'd have to drop out at the last minute due to my pregnancy / birth of the baby / recurrance of any health problems"