In October 2012, Smillie started her own business, DiaryDoll, with business partner and friend Annabel Croft, an ex-international tennis player. Together they created a range of women's underwear specifically for use during periods, with a secret waterproof panel inside them to remove the possibility of leaks and stains on clothing and bedding. They were designed to look and feel like normal underwear – i.e. breathable, washable and not crackly – giving women the confidence to go about their usual activities. DiaryDoll then partnered with charity Endometriosis UK, giving confidence to some of the 1.5 million British women who suffer painful and heavy periods as a result of endometriosis. It was later noted that these were also useful to women in post-maternity and with pelvic floor weakness and the company was rebranded to include all of these groups.