Strange behaviour with small puppies

Hi everyone,

I have a question.

My female amstaff now 19 months old is behaving very strange with puppies only.
She start growling and what look to me a bit aggressive but it is very strange because she never had a fight with any dog , even the dog trainer told me that she is even too friendly , one time she even stopped  a "dog fight" between a jack russell and a doberman pushing away the doberman.
She even play frequently and very nicely with my neigboor jack russell and other like small breed dog.

But when it come to puppies even of her own species she start growling, barking, pushing them on the floor and not sure about biting them as i stop her always before it comes to that.

Anyway i am not sure what to make of this odd behaviour...Anyone have had same experience or may know why?

thanks in advance

Could just be maternal instinct? Suki is very protective of pups.