git-r as has already been said, it is just simple cable stretch, and it is likely to happen to most bikes at some stage or another.
At your mileage, it would be no bad thing that to go back to the shop for an interim check over of the bike.
Also with your mileage rate, it might also be worth you investing in the tools required to remove the pedal cranks and sprocket. I'm only saying this so that you can rotate/turn over the front sprocket, so keeping the wear even, and potentially increasing the life span of the sprocket and chain. It's not a daunting task to do.