Cats growling/hissing at each other

[Sixshot]

My two female cats (biological sisters) are about 9 months old and I took them 2 days ago to the vet to get spayed. Before their vet trip, they were always quite affectionate with me and each other. Since they came back, they've been hissing and growling any time they see each other. It seems to be getting a little better, but still, they were never this aggressive to each other before.

I'm guessing it's just the stress of being to the vet and all, and that eventually they'll snap out of it. I have to keep them separated when they sleep for now because otherwise they'll hiss and growl all night. I forgot to mention, the vet said the cats were already in heat when they were spayed. Don't know if that could be a reason why they're all upset with each other or not.

Does anyone have any advice? Is this normal behavior for cats?

Thanks!