It seems like infected sebaceous cyst
let me explain what do I mean, their are glands(bunch of cells modified to do certain things) called sebaceous glands which secrete the sweat and an oily substance called sebum, they have an opening at one end and closed at other like a bottle normally it can get the production out from the opening but due to one or other reason the opening is closed/sealed and hence the production of sweat / sebum can't come out so it starts building up and it seems like a bump still at this time it will be painless, but slowly it might get infected and the infection makes it painful which is present in your case.
CONSULT Physician and let him know about the problem
As you are already taking antibiotics with out any improvement the only remaining options are
1) Change the antibiotics to higher classes and take pain medication
2) If nothing works they need to make a small cut to drain the pus and collected sebum , may not need to make even a cut some cases they can just put a needle and drain out the stuff.
------- OR -------- Here is what you can do at home if not willing to see doctor.
1) Boil some water and dilute dettol
and clean the area with it.
2) Try to use keep it clean and dry.
3) can apply powdered antibiotics around the wound
4) leave the wound open no closing with bandaid - (as will protect from anaerobic bacteria infection-due to oxygen