It can be due to allergic reaction, viral rash, heat rash
or sweat dermatitis. Chicken pox produces an itchy, blistery, pustular rash
that typically lasts about a week and is sometimes accompanied by a fever or other symptoms. Moreover you are vaccinated for it. Hence it does not sound like chicken pox.
Avoid using any lotions and creams for the meantime. Use a mild cleanser with moisturizing properties like Cetaphil.