I think you were right to bring it up with them, I would be annoyed to, only if they knew though, dont go mad...but in there defence were they aware the dresses were being discontinued at christmas? Having never been a bridesmaid, or planned a wedding before I never knew that dresses took so long to make and that the designers discontinued them.
Glad you sorted it out with them though, I know what you feel about being deflated and upset with them, I had a few issues with my maid of honour to start with and felt she really wasnt interested which upset me, but we sorted it out very quickly, so I know how you feel there!!