I hope somebody can help. My female cat (1.5 years old) took to hissing and atta

I hope somebody can help. My female cat (1.5 years old) took to hissing and attacking my male cat (2 years old) if he did a noise other than purr or meow at me and has for a few months since my ex and I split up and I ended up with both cats. It's like she became overly protective of me. My male cat Loki got out last night and finally wandered back after several hours but my female cat Oona now hisses and growls at him anytime he comes near her. He can just be walking by and she'll start hissing. She started this new behavior a couple of days ago and it has gotten significantly worse since Loki came back.

Other than this, she acts perfectly normal, no increase or decrease in food or water consumption. Bowel function is normal, no signs of pain or increased stress. She still runs around and plays and cuddles like normal it's just she's now turned into a cat-Cujo around Loki and for the life of me I have no idea why. This is my first time owning cats so even though I've had them since they were kittens (Loki was 8 months old and Oona was 8 weeks old when I got them) I'm still learning. They are fixed and spayed and have always gotten along minus Loki exhibiting the same behavior when I first brought Oona home. They used to cuddle, nap together and wash each other, it's like a complete 180 now. Please help!