Just A Hunch?

In chapter 22, Sal is sleeping over at Phoebe's house after Phoebe's mom disappeared.  Phoebe is acting very fidgety about every noise she hears and cannot seem to calm down.  Eventually, after they both fell asleep,  Sal awakes to the noise of Phoebe crying.  When Sal asks her why she is crying, Phoebe simply replies, "I was not.  I most certainly was not."  The next morning, Phoebe does not want to go to school and tells her father that she is sick.    

My question is:  Why is Phoebe denying the fact that her mom left?  Why is she denying help from Sal?  Does she not want Sal to think that her mom left her, like Sal's mom did?  Did she lie to stay home to wait and see if her mom would come home? 

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