Please Do not panic. It could be some kind of injury
in that particular area which could have occurred in case you have scratched or rubbed it. Another possibility is it could be a folliculitis
(inflammation/ infection of root of hair because of excessive rubbing/scratching or some infection) Your narration suggests it to be folliculitis only. There appears and eruption with an irritating pain initially and gradually it gets bigger and pain starts decreasing and once it is ripe the pus is discharged and then resolution of wound happens like any other injury.
So it will heal itself, do not use bandages/plasters. wear loose clothes and try to keep the area free when free at home. Consult a doctor to rule out the spread.
Wish you a speedy recovery to health