It is very possible that you have epididumitus (sp) which is an infection of the epididimus and it hurts like you explained. It is a dull ache that really doesnt stop for days.
You also could be experiencing a varicocele (sp) which is the same thing as a varicose vein in the leg but in your scrotum instead. The vein swells up and grows too long and causes constant pressure to build and squeeze the testicle. In 1991 I had a varicocele ligation while in the navy and then as a post effect I got chronic epididimitus which flares up several times a year.
The infection can be easily treated with tequin or e-mycin or penicilin. If that fixes the pain then you should be good to go. If not they may order an ultrasound to see if you have a varicocele or some other problem on the testicle.
You really need to be seen by a uroligist asap and get this taken care of. You can never be to safe when it comes to the jewels! :lol: