First off, let me begin by saying that I love this image. The sky and character are both very beautiful and correspond nicely. Shading is exceptional, and it seems you have a good understanding of light source. This painting gives me a soft, spirited feeling. Before I continue, I'd like you to congratulate you on being featured with this piece on Transformice!
My criticism would be that the mouse's form doesn't much say "falling", even if that's clearly what it's doing. Because of gravity, and wind, it's most likely that the arms wouldn't be able to press against the sides so easily and would probably float upwards, like the wings and legs. The hair would also be flowing up, like her limbs. This may make the pose more dynamic, as well.
To show more dimension and perspective, I would recommend feathers or clouds in the foreground to prevent flatness in the piece.
I would also be interested in seeing where her gaze is fixated! What exactly is giving off such radiance? If it's not important enough to add in the picture, I'd suggest maybe changing her expression/focal point, so that the audience's attention isn't drawn to something they can't see. However, vagueness is also acceptable! It provides a sense of mystery.
Regardless, I think it's a great piece of art no matter what you do or don't alter! Please don't take my criticism personally, and keep drawing beautiful things!