So my (relatively) new cat was looking out the window. She's a year and some months old. There is another cat in my household, that is about 8. They get along. They both have looked out the window together, and have looked outside of the windows individually. This morning, as she was looking out the window (it was dark out), I heard her growl unlike I have ever before, and then yelled like she was in a tussle with something (again, unlike I have ever before). It was ear-piercing. Sometimes a cat will come onto the ledge of the window. So I looked outside, and didn't see any cat outside, and only her reflection in the window. It is probably one of those. I had her come down, and then she started acting strangely. She walked (or crept) very slow around my place. Both cats tails were flared up, and they were on alert. Then when she came in contact with my cat, she growled at him. And now she has been acting weird ever since (about an hour now). She doesn't let him get too close to her now. She has been getting calmer, but just now, she was not happy that he got so close to her. What should I do? Is there anything I should do when they get into staring contests? I try to break it up, but it doesn't seem to do much.