Females suddenly fighting

Question
I have 2- 5 year old sister cats.  Both spayed indoor cats.  They have always been snuggly and playful but I've never heard them his or really fight.  

2 nights ago I woke up because they were hissing, snarling and growling at each other.  Yesterday they weren't snuggly like usual but no fighting.  This morning again the fighting started.  If one passes the other one or both are on the defensive.

This is my first experience with having cats and I don't know if this is normal or if there is anything I can do.  It also intimidates because I'm concerned that at anytime, I'll get the same treatment.

Answer
Hi Cindy,

Don't worry, spats like this happens. It could be as simple as one of the cats being in the place where the other was. They will get over it. And you shouldn't be afraid of your cats. Remember, you are quite a lot bigger than them. If one takes a swat at you, swat it back. They will learn that you are the top cat in the house. Good luck.

Ciao, Karen