It is hard to tell why birds aren't well. Moulting does not generally show on a bird, in my experience. I normally only notice excessive feathers loose on the cage floor, but the birds look fine. Later, I can see - at close inspection - new feathers coming in.

The birds never fluff up during the moult, not like you're describing. But they fluff all the time for a few seconds for various reasons, to keep warm, or Jake does it to say he wants a bath (I don't know why, it's just his way
.
) or because they're nervous.

Even when the cockatiel was really sick - hours form death with a respiratory virus - he was not puffed up. But then, we knew he was ill and were keeping him warm.

It could be your bird is ill and trying to keep warm, may be injured, or she could have an impacted egg. It may some kind of bug infestation.

Also they sometimes just fluff up to sun themselves - I hope it's just that.