The pain that you are experiencing in the left testicle is probably due to soreness following masturbation. Sometimes if it is done quite rigorously or many times during the day, it can aggravate the pain which then radiates to left lower abdomen. Testicular pain generally will subside if the intensity of the act is reduced.

However if the testicular pain increases it could be due to various conditions like Torsion testis where the testis appears twisted. Then I would suggest you to consult your doctor.