
That first season is their body saying they are physically ready to have pups[not that anyone in their right mind would mate a bitch on her first season]if for any reason they are under the weather, stressed, been some upheaval in their life or under weight this can be delayed.
When is her dam next due in? Will probably find she will come in at the same time, my bitch pup had her first when her dam had her second one after having the pups and hers are 7-8 months apart.
As has been said nothing to worry about just yet.