Summarize:

With epididymitis, pain usually starts on one side, rather than both at the same time. Over time, it may slowly expand to both sides. Typically, you'll notice pain on the underside of your testicle first, though it will spread to the whole testicle.  The type of pain differs depending on how long the epididymis has been inflamed; it may be a sharp or burning pain. If pain comes on quickly in both testicles, it's likely not epididymitis. However, you should definitely still be seen by a doctor. The swelling or redness may be on one side only or spread to both sides over time. Your testicle may also feel warm, and you may feel uncomfortable sitting because of the swelling in the testicle.  The testicle will also appear red because of the increased blood flow to the area and become swollen because more fluid is leaking into the infected area. You may also notice a lump on your affected testicle that's filled with fluid. You may find urinating painful with this condition. You may also feel the need to go to the bathroom more often than usual, or that you need to urinate with more urgency.  You may also have blood in your urine.  Often, epididymitis results from an infection that starts in the urethra and then moves up the tube, eventually infecting the epididymus.  Any infection in the urinary tract can irritate the bladder, causing pain. Sometimes, a clear, white or yellowish discharge may appear at the tip of your penis due to the inflammation and infection of the urinary tract. This symptom is more likely if your infection is caused by an STI. Don't worry. Even if it is an STI, it's still likely to be easily treatable. As the inflammation and infection spread throughout the body, fever can occur as a defense mechanism. Chills may accompany your fever, as well. Fever is your body's way of fighting off infection. Anything over 100°F (38°C) means you need to visit the doctor. Acute epididymitis is characterized by symptoms that have been present for fewer than 6 weeks. Symptoms that stick around for longer than 6 weeks are indicative of chronic epididymitis. Let your doctor know how long you've been experiencing your symptoms, as this may impact your treatment.
Pay attention to testicular pain that begins on one side. Look for swelling or redness in your infected testicle. Notice urinary symptoms. Look for urethral discharge. Take your temperature to see if you have a fever. Keep track of how long you experience your symptoms.