Why Is My Penis Shiny And Slightly Purple?

hi im 17 and have a soft lump at the base of the penis which is kinda shiny and slightly purple. i can move it round freely and only seems to be attached by the skin. it hurts to touch and when squeezed a white substance comes out.
i am sexual active but me and my girlfriend lost our virginity together so dont think its an sti.
is this a serious problem? what is it?