Is it normal to occionally have a painfull testicle?

About once every 12-18 months one of my testicles becomes very painful for a day or two. It is not painfull all the time however, only when it is touched (so it hurts when I walk, prod it etc.). This has never worried me before until today when I started looking up the possible symtoms of testicular pain, and realised that some were very serious.