I can feel my right epididymis but can't feel the left one. My right epididymis sometimes tenders and swells. Is it inflamed? Epididymitis? Treatment?

Infection. It sounds like you may have epididymitis. This is an infection or inflammation in the epididymus (the part of the testicle that holds sperm). This is basically the male version of a uti, which is much less common in men than women. This is definitely worth a visit to the doc for proper treatment.
Yes it is. You likely have epididymitis, an inflammation of the epididymis which surrounds the back portion of the testis and wraps up toward the top of the testis. This is the most common condition matching your symptoms, however you should be examined to rule out other possible and more concerning conditions.