She probably "yawned" - snakes open their mouths widely sometimes as a method of resetting their jaw after eating. They also seem to just do it randomly every once in a while. It's actually pretty cute, heh. The tiny popping sound you described was probably just her jaw.

It can be cause for concern, but only if it is done excessively and with other symptoms like mucus, frothing, bubbles, or popping/wheezing. Pardon me, I'm slightly confused - did she only do it once, or is she doing it multiple times during handling?