Is Merlinko (or any other pet) new to the home? Even if Ninja liked other cats in the past, that does not mean that she will automatically like a newcomer to the home.

I would seperate her to reduce her (and your) stress until you can have her see a vet.

In the meantime, try to think of ANY and all changes in her life, your life or your household that might be upsetting to her.