Ever since I took one of the cats to the vet, my other cat has become very aggressive towards her. Why?

I had to take one of the cats to the vet and from the moment we got home my other cat has been acting very aggressively toward her. It has been 3 days and he is still hissing, growling at her, stalking her around the house and acting like she’s a stranger cat that has come into his territory. The strange thing is that they have always been buddies. He’s never acted this way toward her, he always grooms her and loves her. At first we thought it might be the smell of the vets office on her so we gave her a bath (poor thing, like she hadn’t been through enough already) but that didn’t help at all. If anyone has any advice I would really appreciate it because she is sick and this is stressing her out and making it much worse. Thanks.