Was this inappropriate to wear to a wedding?

I wore this to a wedding: http://www.freepeople.com/Daylight-Duster-Slip/ SECOND PICTURE and the bride was so mad. What do I do? Was I out of line? It was just the white train looking slip with a turquoise shirt.I was wearing a short white skirt under (not longer than the slip), my shirt covered my arms, no cleavage. I also had a long brown cardigan that had the same cut in the back. Was I out of line? Do I apologize, I feel so bad.