If you are experiencing sudden pain in your left testicle then this could be a very serious problem. There is a condition called testicular torsion which occurs when the testicle twists around itself; this cuts of blood supply and can result in death of the testicle if it is not quickly fixed. Usually, however, the pain is quite severe (not just "sore"). The testicle that is twisted is usually also higher up than the other one. If there is any concern about the amount of pain you are having, then it would be best to seek emergency medical care just to make sure that this is not a testicular torsion.
Other causes of testicular soreness could include infections of the tubes leading from the testicle into the urethra. This is called epididymitis and is quite common; it can be diagnosed by your doctor
and requires antibiotics to clear it up. Sometimes, a testicle can be sore simply from frequent sexual intercourse or masturbation.
Cancer is less likely. If you carefully feel all over your testicle, it should be smooth and soft. A hard lump inside the testicle would be more concerning for cancer. You doctor can confirm this for you by examining the testicles as well when you go to see them.