How long has it been swollen for?
I am from the United States.
Swelling that is acute in nature is due to an infection. You most likely have a condition called orchitis. This originates from a urinary tract infection. You need to get into see your doctor for an examination and a urine culture and you will almost certainly need to be started on antibiotics to treat this. Hope this helps and let me know if you have questions.
It is very typical for this to only occur in one testicle. At your age, these types of infections are typically not due to sexual transmission. They are due to urinary tract infections which are probably from not being able to empty your bladder completely. You will need to see a urologist to do a bladder ultrasound determine if incomplete bladder emptying is the root cause of the problem.