calves/shins hurt so much it’s hard to walk

For the past seven – ten days, my lower legs (from the knee to the ankle) have hurt so much I can hardly stand.
When I walk, which I have always done quite a bit, every step hurts.
I want to constantly find a place to sit down. Can you tell me what this problem is and what I can do about it?
I wear Ecco or Mephisto walking shoes, with prescribed (i.e., fitted) orthodics in my shoes.
This does not seem to help.
Thanks in advance for any help you can give me.
I don’t even know what doctor to go to for this problem.