Hi, your calf has some of the strongest muscles in the body, like the gastro-soleus which is used extensively by sportsmen like you. This muscles help you to lift your entire body on your toes and gives power to your leg and foot while doing cardio. Muscle also has two parts -the red elastic bulky muscle and the white strong but less elastic tendon. Tendon attaches the muscle to the bone. It is quite possible that small repeated stress to this area ( micro-injuries) and lead to accumulated disruption of the fibers ( tear) or a single overwhelming force of contraction to the tendon or muscle-tendon junction could have ruptured it.
Tendon+Muscles heal over time. To facilitate healing you must modify your training and avoid whatever causes pain, do gentle stretching to prevent the scar from contracting the muscle. If the pain doesn't go after several days do not hesitate to show an orthopedic. He will be able to correctly identify the problem and suggested remedial action. If you are working too hard you can also fracture your bone ( microfractures = stress fracture
). Remember Prevention is always better than cure