Why is Sophie's mother angry with her after the Halloween dance in the novel, What My Mother Doesn't Know?

Sophie wants to wear a skinny black dress to the Halloween Dance, though her mother disapproves, and convinces Sophie to go with a different dress instead. Sophie, however, secretly buys and wears the dress to the dance, only to be discovered by her mother wearing the dress after the dance. This leads to temporary rift between mother and daughter.