is due to viral infection
u should take antiviral
medicine like acyclovir
within 3 day of infection.
after 20 days there should not any blister as it usually resolve in 15 days
but still u have active blister ,u need to consult dermatologist
and take acyclovir in standard dose
over blister apply plain antibiotic cream like fusidin twice a day
never pop or pick it as it will leave scar and it is difficult to treat .