This was the 2nd time I had her over. The first time was fine.
All of a sudden my female cat attacked her from behind and kept
trying to go after her. She was hissing and growling - it was insane!
Any ideas what could have happened?

Hiya and welcome

That sounds like she is jealous of your girlfriend. May sound crazy but it has been known.

She doesn't have any pets, but claims to love and be loved by animals.

May just be jealousy then hun - how olds your cat and how long have you had her? has it always been just you and kitty?? xx

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Definately sounds jealous to me, my cat will always push me and the OH apart if we are cuddled up.
If you cat is used to all the attention she definately want one someone else coming in and taking up her time with you.

My kitten FREEDOM did the same with me and my partner. Never got aggressive but would always come between us

I've had the cats for about 7 years now. Me and my ex-girlfriend were living together for 2 years and recently split-up. I've been on my own for about a month now, but my ex didn't care for the cats and wasn't very affectionate towards them, so I don't thing "she" is a factor in this. Nothing like this has ever happened before. In fact, we were sitting next to each other at the computer when the cat attacked her from behind. What a scene it was!

Never be too sure - they put up with your other girlfriend not liking them - perhaps theyre worried you're bringing another like her into your family hun x

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Jealousy is probably it. My cat is "very." She desires constant attention. How do I prevent this from happening again?