I'd not read into that too much, as if she's bloated from meds or has gained some weight it might simply not fit.Thing is with the royals is that they know that every single tiny bit of their public image will be scrutinised and commented on. So Kate would’ve known that her lack of wedding ring would be seized upon. Yet she still chose to leave it off. Interesting.
Generally if you are going through weight loss or gain you wait for the weight to settle for a while before re-sizing rings, as it's not good for the ring to be re-sized multiple times.
If she's consistently not wearing it when she's back for appearances, that would be the time for reading into things.