What Causes Left Sided Pain In The Testicle And Buttocks After Having Oral Sex?

I have pelvic pain to the left of my testicles. The pain vascillates and extends down my buttocks, and down to the top of my legs off and on. It is a burning sensation when it does so. It also seems to extend to my penis, and create a burning senseation. I have been prescribed bactrim in concert with naproxen, and some other antibiotics prior. Latest diagnosis was prostititis. Pain seems to only be worsening. I did receive oral sex from someone, and thought this could have caused the problem. I was checked for STD (gonorrhea, clymydia) at that point and tests came back negative. Thank you