i broke a rib and didn't have trouble breathing, so it could be. if you're breathing is okay but it hurts like a motherfucker, it is either 1)bruised badly OR 2)a broken lower rib (lower ones don't affect breathing as much)
mine was a lower rib, so i was lucky in that it didn't affect my breathing. i also think mine was relatively minor, it didn't take me long to get back on my skateboard