It could be a lot of things.....the ones i can think of off the top of my head are
1. injury
2. torsion - testicle twisting on itself.....may twist and then release so blood flow is not obstructed
3. hernia - dull pressure feeling
4. epididymitis - pain decreases when you elevate the testicles
5. varicocele - feels like "a bag of worms", you're feeling the dilated veins
6. hydrocele - fluid accumulated in part called tunica vaginalis
7. spermatocele - cyst full of dead sperm
8. testicular cancer - usually painless or symptoms of dull aching
9. orchitis - do you have mumps? doubtful tho
10. infection - anything down there can get infected just like anywhere else in your body
11. Kidney stone - unlikely bc pain is usually 10/10 located in your lower back and radiates to your nuts
you also feel tired, do you have a fever? How long is the pain present? What makes it better, what makes it worse? What have you done for it? What do YOU think it is? Does the pain move anywhere? Describe the pain.....
Have you been with a new partner lately? Is he/she clean? Any history of stds? Have you had anal sex? Did you receive oral sex and were accidentally bitten?
I dont know what the guy above is talking about, you DO NOT NEED to see a urologist, they will be no more capable of diagnosing epididymitis than a GP.....I am no huge supporter of GPs for many things but its my honest opinion you dont need to see a specialist right away. Physical exam, maybe a couple tests will show if there is an infection and its not a "strong course of antibiotic", its a pretty common antibiotic given for 1-3 days depending on the dose. If its prostatitis, which is very doubtful actually if those are your only symptoms then the antbiotic is stronger, however you wont need to milk to prostate because prostatitis is very tender to palpation (touching).
Again, im not offering medical advice, just giving you information that youd be able to look up on your own