You seem to have a skin condition affecting sweat glands called Hydradenitis
Suppurativa. Usually seen in armpits, perineum. Show it to your doctor or a Dermatologist
. A course of oral anibiotics, zinc gluconate
, anti-inflammatories along with topical Chlorhexidine
solution to keep the area clean & dry should help cure it.
Shaving the area is a good idea. No, its not an STD.