She's been spayed, but two different vets (in two countries) think she's got an ovarian fragment left behind, meaning she still has seasonal hormonal fluctuations, as if she's going through seasons. Of course, she can't get pregnant, but I still don't want there to be any shenanigans!
The vets didn't think surgery was a good idea because of her age, and the fact the fragment could be tiny and have migrated to anywhere in her body. And, of course, dogs don't have the menopause, so she'll go through this until the end.