Hello and welcome to HCM,
A pus filled pimple between vagina and anus could be due to a boil, furunculitis or Bartholinitis.
Bartholinitis is inflammation of Bartholins gland which is present in the area described by you.
Boil is
skin infection and furunculitis is infection of the hair follicle which are abundant in genital area.
The pus filled lesion is unlikely to be due to
STD.
A clinical assessment of the lesion is required to diagnose the condition.
In case, the lesion is due to boil or furunculitis, a course of antibiotic will be required.
In case, the lesion is due to bartholinitis, a course of antibiotic and deroofing of the lesion will be required.
Definitive management can be decided after clinical assessment and diagnosis.
Thanks and take care
Dr Shailja Puri