大姨媽总是防不胜防
[dà yí mā zŏng shì fáng bù shèng fáng]
'Auntie always catches me unawares.' In Chinese slang, 'Aunt' is sometimes used to refer menstrual cycle. So, this is about the unpredictable yet inevitable arrival of monthly period that disrupts one's life, often unexpectedly.