Thanks for your query,based on the facts that you have posted it appears that you have pain and discomfort in left testicle since recently.
This could be due to either
1) Infection of the spermatic cord
3) Infection of the Epididymis
Ideally one should get it confirmed by doing ultrasound scanning of the scrotum.
You need to take broad spectrum antibiotic like Cefexine along with anti inflammatory drug like Diclofenac
It should get resolved within a week.