What Causes Fluid Accumulation In Armpit?

hi,.. i discovered i have an accumulation of fluid under one arm
in the axillary space.. .it is soft to touch.. had it for along time, but just noticed it today.. i had gained some weight and i thought it may have been a skin fold, however today i noticed it is only under 1 arm...no other sx.. should i be worried..can it be drained if its just fluid?