D.N.
It is very normal for a toddler to cry really hard and appear to not breathe or turn blue. I was always told taht if you have to, blow lightly in their face. My daughter sometimes does this and the slight breath shocks her enough to take a breath. All of my nieces and nephews have had bouts of hard crying as well. But if you are concerned or if the slipping appears out of nowhere, go with the test. If anything it will give you peace of mind.