hello and welcome to hcm,
did you had infection in any of the teeth in that area of knot previously ????
i suppose yes or may be some dormant deep caries in the vicinity teeth may be the well reason for this, that you might have not noticed.
might you have taken antibiotics for that or some other oral infection and sometimes due to overdoses of antibiotics, the lump appears in the oral cavity called as antibioma.
go to the dentist
and take some x-rays, so that if some treatment can be performed will decrease the existing infection and fall down the knot size.