Generally they "yawn/gape" after feeding to realign their jaws. I have caught some of mine doing it from time to time randomly .... From what you have described I would just chalk it up to them stretching their mouths .... Doesn't sound like RI, as that is usually accompanied with them periscoping or standing straight up in the air and giving a raspy/gurgling sound. .... I don't think you have anything to worry about