Welcome to HCM,
It seems that there is soft tissue infection
on butt crack leading to pus formation.
As pus is drained nothing to worry.
Go for regular dressing with antibiotic cream for few days.
If there is some induration, go for one short course of oral antibiotic medicine.
Many a time this part is poorly cleaned due to unable to reach this part for proper cleaning while taking bath and due to groove between butts there is collection of sweating and dust, more prone to infection.
Ok and bye.