bloodied chicken

Can anyone help me please I'm at the end of my tether.My normally lovely gentle Tilly hen has turned on the smallest one of the three who has ALWAYS been in charge & top of the pecking order where poor Tilly is concerned.....From nowhere Tilly has turned (like a worm!) she has pecked the head of the small one till blood has been drawn andgone really ferocious!We separated the little one  then put her in with the third one who has also decided to have a little 'go'!......Can anyone help please as Tilly set on her again earlier drawing quite a nasty gash in her neck & more blood.I hate the thought she is locked up with them in the night.Why would she SUDDENLY turn like this?What can we do?It doesn't help that the weather is just raining EVERYDAY....

 very many thanks for your prompt reply in helping us to try and solve this upset with our hens.....I will be off to buy some purple spray from hopefully Pets at Home chain.This morning we managed to separate the pecked bird and leave her in the coop area whilst the other two have the rest of the run but obviously can't get into the coop to lay meaning they have to lay outside...and the weather is SO awful!Why doesn't stop raining??!!!