Could it just be habit she is developing? My step son did some odd things for a couple days (weeks?), and then it settled down. Keep an eye on her, see if there are any patterns (does she do it when she's asleep?) Is it a conscious behaviour or unconsious? Could it just be that she is finally getting some nice satisfying deep breaths compared to pre-surgery, or she is analysing how deep breaths make her sternum feel? OR as the PP mentionned, her throat hurts when she breathes through her mouth? I know My step son's throat hurt for a week or so after his surgery, maybe more.
By the way, I am glad that she everything seems to be going well. Please keep us updated on her progress.