THE MYSTERY IS SOLVED!
OK, it was my mum's and she bought it from one of them Ann Summers parties. She never wore it though.
Not too long ago my sister, 8, and her friend, 7, were playing dress-up and make-overs, and they'd obviously had a look through my mum's clothes. And my sister's friend put it on and they went downstairs to show off their new look, obviously my mum freaked out in embarrassment and told her to get it off quick, and later on my sister must have put it in my clothes box.