Try only feeding peas one day, and then fast them for 3 days. How are you feeding the peas? You should boil them in water over the stove or in the microwave for a couple minutes and then remove the outer skin and smash them on a spoon.
She could just have a lot of food in her stomach. Or she could be pregnant. Livebearers' gestation period is around a month. Not 2-4 days. That would be ridiculously fast!
If you can get a picture of her that would be helpful because we could see if she has a gravid spot. Even if you don't have a male swordtail, if she was kept with males in the same tank at the pet store they could have impregnated her. Livebearers can store sperm from one mating for many months.
