You didn't do anything wrong to your pretty new bike! Like the others said, it is most likely a stretched cable. My bike shop has a policy where they tell everyone who purchases a new bike to ride it for a couple hundred miles and then bring it back in and have the cables tightened again. No matter how much they stretch them before they set up the bike, those darn cables will stretch more as you ride it. If you can't figure how to DIY, you may need to take a trip to your bike shop and let them get it back to where it should be. Good luck!

Annie