I know exactly what you are talking about.....but don't panic..it's called the Epididymis and its where new sperm collect prior to moving up the vas deferens where the sperm mixes with seminal fluid (cum) and is ejaculated.
Now, it's not uncommong to feel a ball like growth on the side of one of your testicles.....thats the Epididimyus and Vas Deferense. Check this site out..it talks about that and how to examine yourself.
http://www231.pair.com/grpulse/bt/lisete.html
If you are able to cum during masturbation without any problems..then most likely it's nothing. However, if you are NOT able to ejaculate or it's painful..get to the DR's QUICK!