How can a guppy in solitary confinement get pregnant?

Live bearers such as guppies and sWordtails have the unique ability to store sperm from male guppies for as long as 6 months or 6 drops (births). If the female has been in the same tank with a male even once, they can get pregnant. If you are sure they have never been with a male, it may be a fake gravid spot, which is not the fry's eyes showing through the skin, but rather the pigment of the actual skin, which makes them look pregnant. Hope this helps : )