Does the chain on brand bike get loose?

Your bike is still under warranty. there could be a problem with rear derailleur, shifter, cette, cette body, or some other drivetrain part. If you mess around too much without knowing what you are doing and break something, your warranty could be voided. Treks are sold by reputable dealers who are trained to take care of your bike. Don't be afraid to go back to them to fix your problem. If it is a defect you are covered, if it is an adjustment they may still take care of it without charge, or even a nominal charge. Chain slack is taken up by the rear derailleur, and chains take thousands of kilometers to stretch enough to cause problems