my two cents... soft tissue, not so bad. ice helps. rest helps. don't push it and end with a strain. probably overworked soleus or gastrocnemius, maybe minor strain. stretch both (straight leg and bent leg calf stretches). stretch all muscles from core down. rest & ice. don't strain it. keep hydrated. stretch. work on mobility. ice. when you go for a run, warm up slowly. A good PT will be able to diagnose a strain and give you more specific guidance.