My Child Is 5 And At The Back Of His

My child is 5 and at the back of his last molar there is a tiny hole behind the last molar on bottom left side the gums... the hole is sort of dark red looking and that side is swollen too and they seem fine I am just concede about that tiny hole ... he just went to the dentist a week ago to get fillings for cavities but only for the top molars ... wouldn’t they have seen it coming before?