If your female guppy has ever been in the same room with a male, it’s safe to assume she is pregnant. That’s just how guppies are. You may draw whatever conclusions you wish about their moral character.
As the pregnancy develops, her belly will get larger and larger (unbelievably large right before she gives birth), and most guppies also develop a “gravid spot,” which is a roughly triangular or crescent-shaped marking at the lower rear of the belly, just in front of the anal fin.
A female guppy will give birth about every month. Gestation takes from three to eight weeks, depending mostly on water temperature, with four weeks being about average. Your guppy may well have already delivered fry, because the minnows and frogs, as well as the guppies themselves, would quickly eat any guppy fry that appeared in the tank.
I have not ever seen a female guppy that has been any where near a male that was not pregnant. Yes, every 28 days. When they drop they go to the bottom (they are fish food). Those not eaten recover from the birth and swim to the top (more food). If there any survivors, they will grow up in 6-8 weeks to start their own families.