They have a jacobson's organ that the use in order to breathe while eating. May have been taking a little break if it was a large meal. Sometimes you can see it, it looks like a little tube sticking out from the bottom of their mouth.

If I remember correctly, snakes have a large and a small lung. The large lung they will shut down while eating a large prey item and use the small lung to breathe.

Lesson learned. That size prey item was a little too big.