The triangle is called a gravid spot. As everyone else has already said, it is an indication of pregnacy, those are actually the eyes of the fry. With my sunset platy I could 'see' some of the babies. Congrats! If you want to keep the babies you'll need to add lots of plant cover for the fry, otherwise the adult will eat the babies. A big ball of java moss would work well for this or you could get a grow-out tank.