Replying to Different foot board hardness in FT

So yeah, I've recently had some pain in my left arch while skiing.. Boots are full tilt hot doggers, the red ones. So I thought that I would examine the boots a little to see if there was any difference between the two, and when removing my boot boards I realized that the left one (the foot hurting) is way more stiff/hard than the right one.
The letter on the left one is "S" and "D" on the right one.
Could this be the cause of pain? Is it supposed to be like this? What should I do?

best, axel