Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern about Janet. It sounds as if she may have had a focal or petit mal seizure given the length of time the episode lasted and then she was perfectly fine as if it hadn't happened at all.
These episodes tend to occur totally randomly and are completely involuntary. However, when they start to occur in a cat this age, then I always worry that there may be a brain mass which is triggering the episodes. Unfortunately, it would take advanced imaging such as an MRI or CT scan to confirm although these lesions aren't typically located where it's either advisable nor practical to remove them.
I've also see a few cats with hypertension behave in some odd ways. High blood pressure is rarely primary in a cat but usually secondary to either Hyperthyroid Disease, diabetes or kidney issues. You mention that she has a low thyroid but I assume you meant that she has Hyperthyroid disease since a low thyroid is extremely rare in a cat. But, if she hasn't had her blood pressure measured recently, then it might be something to discuss with your vet.
At this point, since she only had the one episode, I would continue to monitor her but I wouldn't start her on any medication unless or until she has other ones which are occurring every day or frequently.
It's possible, I suppose, that this was an isolated event but I'd be prepared that it may not be; she may have others although when that might be is not something we'll be able to predict.
I hope this helps. Deb