This will need image of your area which has pimple. For such kind of pus forming boils there are certain medications

What is MRI inference...? If not done get it done. This is acase of patent Uracus. May need surgery.

Hi since ur telling its recurrent U can get ultrasound abdomen pelvis or a sinogram to rule out umbilicus sinus or any sinus tract As umbilicus area (navel)is more prone for infections better to clean the area dry n clean

It can be pre  infective or superimposed infection on some other pathology, either you took incomplete antibiotics course without follow up of you doctor, if their is significant discharge is there , it needs examination, f/b usg and incision and drainage if localised collection is present and bacteriological and hpe of tissue, as per clinical context.