The description is not complete which will help in confirming a diagnosis but it can be allergic contact dermatitis, heat
rash, sweat dermatitis, hives, folliculitis, cysts etc. If the lesions are itchy
then taking oral antihistaminics will help. Also wear cotton clothes and consult a dermatologist for confirmatory diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided.