I need help. My two cats - brother and sister who I have had for 6 years and who until this past Sunday were the best of friends have begun viciously fighting and I have had to keep them seperated for the past 3 days.   There has recently been a stray cat outside of our house and the vet seems to think this could have caused this.  But I have not seen then cat for 3 days and they are still fighting.  The female just growls at the male and he attacks her - he won't just leave her alone. I just purchased the feline diffuser thing hoping to help calm them.  Is this normal?  What else can I do?  I ma extremely stressed out over this - I can't imagine having to keep them seperated forever.

Please help!!!!!