Very unusual! Unless you have suffered the trauma of a puncture injury this would seem not to make sense at all.
It could be linked to the way an ingrown hair wound has repaired - but that would be a pure guess.
what should i do ?
from up close it looks like a deflated pimple or ingrown hair because the surface is pinkish
The only people to make sense of this would be medical professionals such as your own family doctor / GP, or even a Dermatologist