You are so young! I agree that this should be retested again and if this is common for you, that it is problematic. Tell us more about the situation.
Is this 140 a fasting level? If it is a fasting level it is not normal. Normal fasting should be lower than 95, and ideally even in the 80s.
Are you overweight or any other health issues? Family history of diabetes?
I would strongly recommend that you visit your doctor and tell him or her your results. They should do some further investigation. Hba1c and glucose testing.
If you have a home glucose meter, please do some more testing. Test fasting for a few more days, and testing 1.5 - 2 hrs after your biggest daily meal (should be less than 120 to 140).
Please come back and update. Ask any more questions.