不是我们的⼤姨妈。 (Bù shì wǒmen de dà yímā) No, it’s our period.

⼤姨妈 (dà yímā) — (euphemistic, slang, humorous) “Aunt Flo”; a personification of the menstrual period / menstruation.

https://en.wiktionary.org/wiki/%E5%A4%A7%E5%A7%A8%E5%AA%BD

Screenshot from: The Day of Becoming You https://www.imdb.com/title/tt14792350/