What Causes A Pus Filled Lump In The Armpit?

my armpit was itchy after a shower and i noticed a knot like lump under the skin, so i lightly squeezed it, than a smelly, white/creme coloured discharge came out, it smelt terrible. The smell was overwhelming and led me toanother shower. There was no discomfort, or pain from the knot and there was no raised bump.. what is this?