Hi thanks for your question.
Most probably you had allergic reaction
to some powder/cream or spray that you have used.The rash
could be out come of allergic reaction.Rash on right side responded to treatment but rash on left side got superadded bacterail infection with puss formation.
I would suggest you to consult a physician who may drain your puss,if needed and may prescribe you antibiotic for infection in armpit.
Hope this answers your question.