I'm not sure if everybody will be able to relate with that last panel, but I sure can.
This should be consistent with what's previously been established. For one, people have changed without noticing before, and though Justin did feel strange initially after transforming at Grace's Birthday Party, it was more placebo effect than direct side effect of the change.
Also, much like with change blindness, it's not guaranteed that a person won't notice they've transformed right away due to various x-factors, so there! HA!
There's more that could be said about this, such as the implication that muscle memory must be affected and such, but really, I wasn't sure if I was going to bother clarifying this at all beyond mentioning "self transformation blindness". It's a bit technical and difficult to fit in to the comic naturally, though I defend it with Pandora via the assertion that she (and probably most immortals) talk to themselves a lot.
Ultimately, it was the last panel that sold me on going ahead with getting a bit technical about it. I know that feeling, Pandora. I know that feeling.