Welcome to the forum. Thanks for your question.
This isn't epididymitis, and it is unlikely you have any STD from the exposure described. The first need here is for you to see a doctor or clinic for professional evaluation. Self-assessment of one's testicles, especially by non-professionals, is highly unreliable -- especially since minor differences between testicles (in contour, sensitivity to touch, etc) can be entirely normal.
Further, texually acquired pididymitis doesn't cause the vague and mild sort of pain you describe, and is almost always associated with urethritis -- urethral discharge and often painful urination due to the underlying gonorrhea or chlamydia. Finally, oral sex has never been reported to be associated with epididymitis, in the absence of gonorrhea.
I'm most supicious of genitally focused anxiety. Before you assume I'm implying the pain is "in your head", be assured I know it is not. Here are a couple of threads that discuss this possibility, including the mechansims of pain.
http://www.medhelp.org/posts/STDs/Epididymitis/show/2011438
http://www.medhelp.org/posts/STDs/Worried-Sick/show/2069361
So as suggested above, I definitely recommend professional evaluation. I'm confident no epididymitis will be found, and no STD.
Best wishes-- HHH, MD