What Causes Pain And Hardening Of Calf Muscle?

My calf muscle is hardening and is very painful if touched even gently. It does not hurt while walking or using the muscle. I had surgery to remove a heel spur on that same side about three months ago and started working again for the first time after the surgery about one month ago. I stand all day at work and have also started golfing again just recently. I noticed the pain and hardening about 10 days ago.