Premature greying for 3 year old black lab.?

Her undercoat has turned all grey within one week majority of her body is now grey undercoat... im not talking a few grey hairs around her chin and nose im talking behind her ears.. neck back like more then 1/2 her body she's a 100 dog so she big and most of her is gry... she isnt even three yet it's all happened within a week or so? anyone ever experience this with ur lab???