Welcome to HCM.
Most likely it is due to
hair root infection called
folliculitis.
It usually occurs after blockage of perspiration.
You have to
take proper antibiotic like amoxy/clavulinic to subside infection and
nsaid to subside inflammation.Take proper local care.
Clean it with anti septic liquid.
Usu antiseptic soap to bath.
Consult you treating doctor for further guidance and proper medical management.