It is normal, some do it some don't. If they do it prior to laying normally it is a sign they are pretty close.
Also keep an eye on them, because I had a friend last year who had one twitching for a week or so and he had no idea why she was doing it before laying, she had laid but it was a small clutch and he had no idea she was on eggs.

At 28 days though don't worry it is normal.