What Causes Pain In Left Calf When Inactive?

I wake up each night with pain in my left calf only which radiates to my lower back. The pain goes away in the AM with activity.I have recently been diagnosed with slightly high potassium. I am to have it rechecked in 6 wks to see if it has come down. I am 61 years old. No other medical problems other than osteoarthritis for which I take Celebrex.