Mother cat attitude with her two 6 mo. old kittens

Question
We had our two 6 month old kittens spayed today.  When we brought them home tonight, the once loving Mother cat growls, hisses, and swipes her paw at them when they get near to her.  The Mother Cat and the 2 female kittens have been so close.  They sleep on top of each other and eat out of the same bowl and the three of them play with toys together.  They give each other a bath and have been so kind to each other since the birth.  The kittens keep trying to approach her but she won't allow it and rejects them.  I have tried to pay more attention to the Mother but she still is mean to them.  Thank you so much for your help.

Answer
Dave,

Cats know each other by scent. After cats/kittens have been in surgery at the vet, they smell differently.  In a couple of days the scent from the vet's office should wear off and mama will, once again recognize, the kittens.  Normally, when cats encounter cats or kittens they do not recognize they begin the "getting to know you" ritual which starts out with hissing, spitting, and swatting.

All should return to normality in a few days.

Best regards... Norm.