She would be menstruating and not praying, while lying down beside the prayer place of the Messenger of Allah ﷺ as he prayed on his small mat, and when he prostrated, part of his garment would touch her.
أنها كانت تكون حائضا لا تصلي، وهي مفترشة بحذاء مسجد رسول الله ﷺ وهو يصلي على خمرته، إذا سجد أصابني بعض ثوبه.
Bukhari narrated it in the Chapter on Menstruation (333), and Muslim in the Chapter on Prayer (513), both through Sulayman Shaybani, from Abdullah bin Shaddad, who said, "I heard my aunt Maymuna." Then he mentioned the hadith. The wording given is that of Bukhari. In another wording from Bukhari (379) and Muslim, "The Messenger of Allah ﷺ used to pray while I sat facing him, and I was menstruating, and sometimes his garment would touch me when he prostrated." She said, "And he used to pray on a small mat."