As you have asked if it could be in ingrowing
nail, I presume that it is on the edge of the finger
nail. If that is the case, sometimes we get "back friends", little bits of dry skin and when they are pulled off an infection can occur. Sometimes can just happen. You will need an antibiotic cream to apply to that part of the finger.
Your doctor will decide whether an antibiotic cream will be OK, or whether you need a course of antibiotics.