
It might just be she's showing earlier as her muscles aren't as firm as in a maiden. I noticed this with my cats, that second and subsequent litters they seemed to show earlier.
However - I don't know if it's the same with dogs, but in cats (the breed I had anyway) the first litter is often smaller in number than later ones. A first litter of 5 might well be followed by later litters of 6-8. That is a lot for cats... and it was a breed known for large litters; in fact the record for the largest litter of kittens ever used to be 14... I almost fainted when I saw that my first girl had an ancestor from the same cattery as the record holder - quickly did some research and no, not related (phew!) but she did have large litters, 6 in the first and 7 in the next 3.
Be thankful you don't breed rodents. Despite their size they frequently have litters of 16 or more; sometimes as many as 20.