The problem probably occured because you have given your elbow a severe blow,due to a fall or other injury whislt at training.
The nerve passes behind the elbow and through a tight tunnel(the cubital tunnel)at the inside of the elbow
An injury such as dislocation or fracture can tear or inflame the ulnar nerve.(does'nt sound like it in your case)
The inflamed nerve can swell and become trapped (ulnar nerve entrapment)
Prolonged pressure on the nerve can also be a problem.
