Sounds like she may be pregnant and about to give birth. I used to breed and raise guppies and know that if the female is hanging out around the too of the tank for a few days, she's about to give birth. If you know she's not pregnant, then it could be lack of oxygen in the water. If the tank isn't cleaned out enough, the old food flakes and their poop will start to contaminate the water and lose oxygen in the water from decomposition. Also make sure the tank is large enough to accommodate the amount of fish you have.