Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry that Nika is behaving as you describe.
When an older dog starts to walk in circles, unfortunately, there are only going to be few reasons why which I'll list below. Any one of these conditions could develop quite suddenly with no prior warning;
1. There's a central lesion or mass in the brain (a brain tumor in other words) although it would take advanced imaging such as a CT scan or MRI to diagnose such a problem.
2. Inner or middle ear infection although these dogs are typically tilting their heads. Again, it would take advanced imaging to diagnose. However, since most of my clients can't afford such testing, I'll often treat my patients with antibiotics without knowing for sure that this is what they have.
3. Dogs can develop strokes which can affect their vestibular system but these events aren't nearly as common in dogs as they are in humans.
The panting that she's doing may not necessarily be related to her walking in circles although it might reflect confusion especially if she has a brain mass.
And, your other dogs may be sensing that there's something "wrong" with her which is why they're reacting as they are.
Diarrhea isn't typically seen with any of the above conditions, however, so I'm at a loss to explain everything she's doing with one condition.
A bland diet of boiled chicken and 20-minute white rice may help. I'd also give a daily probiotic such as Forti Flora which is available at many pet/grain stores or online.
If she continues to behave this way, then a vet visit may be prudent.
I realize that my answer may not be what you want to hear but I would be doing a disservice to both you and Nika if I were less than truthful and honest in my response to you. I hope you understand. I also hope that this is helpful and provides possible explanations for her behavior. Deb .