Why are my fish nipping at each other?

They are constantly bitting at each other and it's concerning me. Each day it the other one bitting the other. They are 2 guppies I got from petsmart from the same tank. There currently in a 1 gallon tank together. I lve had them for about 3 weeks and they just started doing this about 3-4 days ago. And I have no room in my house to upgrade to a bigger tank. What's wrong with my fish?