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