Sudden agression problem

Dear Dogs Today Think Tank,

My five-year-old Staffordshire Bull Terrier bitch has always been a kind-hearted dog who has never show any aggression to any other dog.

But, every time she sees this small Jack Russell, she goes into a very scathing growling teeth showing aggressiveness.

The only different thing about the Jack Russell is that it has big lumps on the side of it’s belly, the owners say they are tumours but are benign.

My question is why has my usually placid bitch who has never shown aggression to any other dog suddenly has developed a hate for this poor little dog?

P.S. Her son, who I also have, is fine with the dog when we have met.

Kind regards,

Simon John Gibbons, by email