Maki-dono is here to weigh in on this vision of happy panties.
On the animation: HELL IF I KNOW ANYTHING ABOUT THAT. I'm a total novice in animation, I can see it smoothly in my mind and then on ImageReady it turns into Parkinsons eating my frames. It's true that you do have consistency and that is wonderful because it's so difficult to keep lines steady, I find. If we really wanted to dive deeper, I would say to think about detailing the bounce back of the hair and bottom of shirt right before the loop repeats.
On the anatomy: I did want to mention that the chest line (right hand side) is not usually so outlined. It's more the bottom side to show the weight and plus because without the shirt (I'm sure that's a riveting thought) that line would connect lower anyway. So with that follows fabric lines and how they fall vertically..blah blah blah, Google can provide good reference.
Very nice job on the frame work and making sure they matched up, though the snimation itself seems a bit too slow, I'd experiment with speeding up the FPS and maybe adding more frames, to make it even more fluent and such.
Great job for a first attempt at a drawn animation. I sadly have a ways to go before I'm ready to try this type of thing.