Helen,
How wonderful the lady spoke to you as another lady who was about to make the cardinal sin . Maybe it's one reason that more women wear trousers .
I know my wife , daughter , sister in law , mother inlaw have all made this mistake . My mother in law decided to go one better when she joined us in a family holiday , she was having a lovely swim in the pool and playing with the grandchildren , she was scooping them and and twirling them round in doing so she had scooped the neckline of her swim suit revealing most of her top half , I'd never seen her duck her head under water before but she disappeared to the bottom of the pool to recover her dignity !!