Hi Welcome to Health Care Magic......
From what you describe they seem like they are pustules i.e collection of pus.
Causes can be many - precautions to be taken are 1)Keep that area dry 2) Remove the diapers immediately after they get soiled. 3) Clean with mild pressure dont apply too much pressure as baby's skin is too fragile.
Consult a pediatrician
and get treatment with an antibiotic.