I'm worried that she'll look like she's catching her breath instead of relax/normal breathing :( 
I did looked at other idle pixel animation: raising of the heels, arm movement and hair bounce. I tried to make a simplified version which i ended this result 
 

Why not create the motion in her chest then, instead of creating the motion in her legs?

It's supposed to be a idle standing animation ^^;  i didn't do the knees bending and the other stuff cuz she's just standing and breathing. cuz she doesn't have an actual feet drawn on and  there's no heels, i used the legs instead for her to attempt mimicing inhaling and exhaling animation. 
 

It looks very weird that her legs are lengthening and shortening. When people do that action bouncing motion, the vertical motion is created by the knees bending. The legs are set apart, and the person shifts their weight from one leg to the other.