Why are my calves stiff and sore in the mornings when I get out of bed?

The muscles in the back of my legs are very stiff when I get out of bed. I am 51 years old , no health problems , no cramps in the night , I exercise moderately , play tennis twice a week and walk as much as possible.This seemed to begin when I had an overactive thyroid 4 years ago , but that is resolved and I have had no problems with that for 3 years. I have mild arthritis in my lower back. I wear heels and flat shoes in combination.Any ideas please?