If they have been in a tank with males they are pregnant lol guppies breed like crazy lol
So, so true!!! The female guppies were likely together in the breeding facility and separated before bagging for sale, but they were together with male babies before that and why / how they can be pregnant.
Even one male fish mistakenly gets into the females group, babies!!!
Even tiny females just bigger than babies will be 1/3 inch and preggo. It was how I could tell which fish were males, and they all basically looked like babies, except some were gravid and I would separate them that way.
Maybe the one gave birth. Babies are tiny, often especially from young / brand new moms, though sometimes the new moms will give birth to few but larger fry.
I would purchase some baby fish food.
But yes if you have a female guppy, 99% or more it is going to have babies in around a month, gestation is appx. 28 days.
The momma will get shiny and the front of the body will look squared, and then more squared and maybe even more squared and birth will happen.
You will notice she will be going around floating plants, floating plastic plants, live stem plants, or corners of the tank if there aren't tall plants they can hide / get cover in.
The additionally crazy thing about these fish is that the females can store sperm from the males, for months, yes, 6 months or so if it is an adult female.
Livebearers which Guppies are, have hundred of baby fish per year but in nature a huge number of them are eaten by predators, though in a tank the mothers and fathers may eat some of the babies but you will end up with babies most of the time. Now if you had a hungry tankmate, they also might eat the babies. Having moss or plants in the top of the water would be a help for the babies to hide out.