I had asked my mother the next day as how she knew I was not asleep and that I was in pain when she said it was an instinct that she cannot explain.

查原文，是不是漏掉了 to：

I had asked my mother the next day as to how she knew I was not asleep and that I was in pain when she said it was an instinct that she cannot explain.

as to = about, concerning：关于，对于，至于。后接名词、代词、动名词、从句。

【翻译】第二天，我问妈妈，对于我没睡着她是怎么知道的；当她说这是她无法解释的本能时，我感到很痛苦。