Thanks for writing.
As far as your history is concerned, i think the spot is due to some minor trauma during the extraction procedure. As you were on the local anesthetics, you would not have felt it then.
It is sometimes seen that local trauma , at or near the site of work, may cause just a minor bleeding below the membrane which forms a clot there.
Dont worry, It usually will go off by itself or dissolve within a week. if it persists, you will have to visit the dentist
and take further guidelines from him.