Just noticed a brown blotch under the nail of my pinky toe. runs from the matrix at 45 degrees a couple mm up the nail bed but not all the way. what?

Brown nail: could be micro trauma to the nail from pressure from shoes or running. This causes bleeding in/ under the nail. Sometimes there can be "moles" under the nail. Would suggest you see your local Podiatrist or Dermatologist for a complete check.

Differential dx: would include, a bruise, fungus, normal variant, melanoma…might want to see a podiatrist or dermatologist..