Cane, you have some beautiful dogs!!!! Many people online have the same question, so it's definitely a "thing" with older, female, spayed dogs. Women who have had all their reproductive parts removed also tend to have hot flashes during menopause, so I don't see why a dog can't have the same thing.
I couldn't find anyone asking the same question about their aging MALE dogs, so I think it's safe to say that it's a female issue and it's a definite possibility she's having hot flashes.
Did you know that medical experts still can't agree on what causes hot flashes in women~even the Mayo Clinic has no sure answers, let alone knowing if they would happen in a dog or not?
I'm betting if men had hot flashes there would be extensive studies on the subject and every resource would be exhausted in finding out the exact cause and in developing a treatment for it.
Until then, I'd lean towards her aging hormonal system causing a deficiency here and there, leading to hot flashes, which are nothing more than a thermoregulation problem.