Policy on picking up shifts

Does anyone know where i can find the policy on picking up shifts at other stores? My manager is giving me a hard time and i want to know if this is just an ego thing and how much of it is true. I was only scheduled one day this week and picked up hours at a different store, my manager says that i should have asked her for approval first. But I assumed if im not scheduled at my store i am not needed. And if someone else has the hours to give i would be fine. Especially since there isnt OT involved.