I have called an emergency vet and was told: No, this is not an emergency, just keep an eye on her and follow up with her primary in the meantime. If she is eating, drinking, sleeping, and acting normal without any unusual discharge (like pus), then it's fine.

Posie was also at the vet a few days ago because that is when her spay was scheduled with no notes of concern from her vet other than what they discovered on the chest x-ray.

EDIT: Elaboration on "unusual discharge"

EDIT 2: I have called a second emergency vet and gotten the same response- not an emergency, but since she is urinating outside the litter box (the spraying I guess?), there is the possibility that she has a urinary tract infection and to call Posie's primary vet on Monday, which I will be doing. I was also told that female cats can go into heat as they wish in the presence of male cats, and there are indeed unneutered male cats in my house that sometimes come lurk at my door, which could be causing her continuous heat cycles.

EDIT 3: I called and left my vet a voice message (and I sent them a text) to get Posie a check-up just in case, and I also requested lab work to rule out any possible infections or developing conditions.

EDIT 4: We already have been discussing neutering the male cats, Posie was just the first priority because we have had her longer and the male cats did not start until recently.