(in Britain) a formal assertion by a worker to an employer that absence from work for up to seven days was due to sickness. From 1982 this replaced a doctor's certificate for the purposes of paying sickness benefit
See also sick note
Examples of 'self-certification' in a sentence
self-certification
For periods of sicknes shorter than seven days, self-certification is usually required by employers.
Times, Sunday Times (2014)
So in your case self-certification is all that is required.