Onychophosis is the most common cause of pain in the nail sulcus. It is commonly mistaken for an ingrown toe nail. The pressure from the nail results in a build of of skin and it is that thicker skin in the nail groove that causes the pain.

The treatment of onychophosis usually will need the skills of a podiatrist to expertly debride the callused nail groove and remove the painful skin.

Resources:
Not an ingrown toenail? (Podiatry Experts)
Onychophosis (PodiaPaedia)
Ingrown nail or Onychophosis? (Croydon Total Footcare)
The side of the toenail hurts, but its not ingrown (Podiatry FAQ)