Why did my Tetra Kill my fan tailed guppy?

okay, when i Got the guppies i had some neon's and another guppy in the tank soon after the guppy started to give birth but she started to kill the other fish, the guppy was still alive till today. I had 3 tetras and one of them died the other day and i was think okay a fish died nothing out of the ordinary, but today one of my other tetras (the fat one) was picking on the guppy and killed her. now its just the two tetras. theses two have a history of swimming around each other but now their by the plant shaking their tails. I don't know what to do, is my tetra giving birth or is she just being aggressive? the two fish left get along fine with no problem. whats going on????