Congratulations and welcome to the forum! Opening and closing their mouths is normal. They do it when the swallow. You mentioned that it shed right after... mine always start their shedding process by opening their mouths then puffing up. Pacman frogs eat their shed. This is just normal frog behavior.

Like MeTree said, check out the care sheet if you haven't already. Never hesitate to ask any questions! People on this forum are very friendly and we just want to see healthy frogs and happy frog-keepers! Good luck!