Are my guppies fighting or what?

A week ago, my roommate and I got two male guppies and they currently live in a one gallon filter tank. The guy at the store said since they are very small they should be fine and they are friendly fish. BUT today, out of nowhere, they seemed to be fighting. They chase each other around sometimes and look like they may be taking nips at one another? Non of them seem to be injured but I am not completely sure. What should I do? Will they be okay for the night?