A one-month-old baby has redness around the anus with small blisters, possibly a diaper rash. How should it be handled?

After your baby receives the DPT combined vaccine (boosted dose), redness appears at the injection site after a bath last night. Today, the redness has expanded to about 5 centimeters and there is a warm, tingling sensation. Is this a severe symptom? Do you need to seek medical attention?