Nor do I know what to do about it.

First off, he runs away any time I go near him now. This is a big change from a couple of days ago where I was holding him in my lap most of the time and he was purring and loving it. Also, he's fucking with Siren more. He's up all night bothering her and keeping her awake, and he keeps jumping on/at her. They were both getting along fine up until this point. I don't know what happened.

And he was making some weird fucking sounds last night. He was basically chirping, for hours. I never heard a cat make that sound before. He sounded like a bird, basically. I'm more concerned about his interactions with Siren, though. At a few points last night I heard one or both of them hiss at the other one.

This really isn't good. I kept him because they were getting along so well. If they don't get along I can't keep him because I can't keep them separated unless I lock him in the bathtub again.

Any ideas on why he's being so bratty all of a sudden? He was so good for the first week! Why all of a sudden is he fighting with Siren and running away from me? And how the hell do I remedy this situation?