What happens when Jonas experiences his first really painful memory the giver?

Technically, the first memory of pain Jonas receives is sunburn, which is in continuation of the joyful memory containing sunshine. However, Jonas feels only a diminutive amount of pain.

The first memory of strong, intense pain, is the memory of the sledding accident, which follows the pleasant sledding dream. In this sledding accident, Jonas breaks a bone in his leg and scrapes his face against ice. This was quite a harsh pain.