So do I understand this right, she wants to withdrawl the contract all together for you to shoot the wedding and find another photographer?
You delievered 13 images...how many did you shoot total? I only ask because of a previous comment that mentioned that she probably head your shutter cliking away and was expecting a lot more images. I've run into that while chasing a kid around for some candids. The kids mom heard me firing away for a lttle under an hour and in the end I gave her something like 35 images to choose from. Her first response was, "That's it? I could of sworn you took more that 35 pictures." Of course I had to explain that there were doubles, some missed focus since her son was running around, and some that were just flat out not good.
I'd tend to agree that even though they're unhappy with your work the fact that she wants you to shoot the bridal shower is strange. Maybe they want to "give you another chance"?