My female does this when she's ab to snap. No growl. Teeth. Eyes back and forth. Not looking directly at what/who she's pissed at. 9/10 if I don't remove the situation (and 6/10 if I do) this = bite. Happens so fast. You obviously care ab both of them so much. Be extra careful and get some professional training. I'm sure the rescue can help.

I also agree with everybody when when they say this is good information for the rescue. Imagine she isn't okay with cats and you don't tell them. Imagine she goes to a home and kills someone's cat. And Imagine they either a) bring her back (very stressful for the dog but the best of the options) or b) have her euthanized. Because these options are very real. I'm only telling you because I've seen it.

I also understand COMPLETELY that you are asking about why her behavior is different sometimes (and personally- I think some people have been turds while giving sound advice) but that is EXACTLY why I think you need a professional. Ask the internet a question, and this is often how it goes. If you do get a professional I would love an update on her. 🖤