What do other parents think?  Did I not recognize the situation quickly enough?  Should we have gone to 911 as soon as she thought that he "didn't seem right"? 
It sounds like you responded appropriately to me.  I guess a significant factor would be how well you know the sitter.  I've learned over the years that no matter how insignificant my children's symptoms appear to be & no matter how incapable my wife is of verbalizing what she thinks is wrong, I should stop and pay close attention when she tells me something isn't right.
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"There is not a man of us who does not at times need a helping hand to be stretched out to him, and then shame upon him who will not stretch out the helping hand to his brother."  -Theodore Roosevelt