She did have her arms held at her sides basically all the time prior to this while she was walking around, so I guess she didn't realize that they had more use than just holding food until now.

It's interesting to watch Shin-Goji here. She's finally noticed that somehow, she's different from how she was. The implications here are twofold, as not only has she realized that her body has changed, she's realized anything at all. Shin Godzilla lacks a lot of the more traditional 'zilla traits that related to his cunning and will power. It has never been implied to be anything more then a giant moving thing, maybe somehow even less then a monster. You can see that mindlessness early in the comic, where she didn't do anything other then go forward. After that, everything was just a reaction. She attacked the Cerulean because it attacked first. Then she attacks the hunters because they get near, and doesn't really stop until she's exhausted and Monkey can get close enough to share some food. She starts the next chapter nearly as mindlessly attacking Ibis. All of her interactions with other friends and the world around her are characterized with violence. It isn't until Brown Bear deflects her beam and has a straight shot at her face that she begins to change, and even then it's just toning down her offensive. Even here she's mostly just going along with the Hunters because they have food and she doesn't think to do anything else.

But here, after a conk to the head, she takes a look at herself. That's a level of awareness Shin Godzilla never demonstrated, and you can almost see her trying to puzzle out why this is different. I'd love to really know her thoughts right now, what's sparking off in her brain. I'll admit I could be reading a bit much into these panels, but it feels so right. It's the frown that doesn't reach her attentive eyes I'd say, focused strongly on her hands. She looks very questioning.