Question about Testicular Pain?

I have never had testicular pain, but all of a sudden yesterday, I had got an acute and severe pain in my left testicle. It is extremely sensitive to touch, and the pain worsens when I stand up or walk. I almost doubled over in pain a few times. I had something similar to this 4 months ago but it went away after 2 days. Do I have some sort of infection? It only hurts on my left side and I also feel constipated even though I am not. It is not a STD because I have never had .