Today I did the same workout for my back that I did last week and it didn't tire me out as much. Even though it was intensive and quite brutal. And, this worries me because I don't think I'm going to be sore tomorrow and therefore I'm not going to build all that much muscle after this. So, have I reached a plateau already? Because I just started, but I have already gained about 5 pounds of hopefully muscle, since I've been eating right and working out nearly every day. So, should I just up my intensiveness during my workouts and bust my plateau or should I just keep this up, and it will help me build muscle mass anyway?

Help Appreciated, :)