
Bitches go through the same hormonal changes whether they become pregnant or not, sometime lat in false preganancy these hormones reduce quickly whereas in the pregannant bith they do not and she goes on to give birth and lactate.
So it shouldn't be noticed by the bitch that hey she was pregnant and then she wasn't, and in fact bitches that did get pregnant can havew absorbed pups up to 6 weeks in whelp.