Does Regular Running Cause Severe Stiffness In Calf Muscles?

I've been running for about 18 months, I'm 53 years old, average weight. Last few weeks I've had severe tightness and fatigue in my calfs, so much so it's become really difficult to run. Previously I was running 5 - 7k 4 to 5 times per week. What's caused this severe stiffness ness in my Calf muscles?