Corpus Luteum
Human Ovary, by euthman
Where does progesterone come from?
After an ovarian follicle ruptures and ovulates, it becomes a transient structure called a corpus luteum, which is Latin for yellow body. The plural is corpora lutea.
The corpus luteum produces progesterone. After ovulation, there are several corpora lutea present and they will produce progesterone, again whether or not the dog actually got pregnant.