New aggression

I've had my cat for 1 1/2 years since she was a kitten. She is spayed and she has always been around our dog. The two used to play together untill 2 days ago. She flipped. The cat now chases the dog in a fierce frenzy. She hisses, screams, claws and bites. The dog cowers down to her now. I'm not sure what to do. She has always been a sweet furrball that purrs when you call her name. Just wondering where this aggression could have came from and how do I stop it?

Did the dog maybe change the way it smells somehow? Maybe rolled around in some dirt or grass that had some weird smells? I know that sounds like a weird question but cats are very keen on scents.