Cyst Between Armpit And Breast. Bleeding, Pus-filled. What Is This?

i have a cyst between my left armpit and breast that I was able to soak and bring puss to the surface. the cyst started as pea size then became as big as a half dollar....when it did surface i gently squeezed it and a fowl smelling yellowish puss came out then soon after blood...i let it drain a bit then put a prescribed antibiotic on it and a clean, sterile gauze ...i noticed that it is still draining and just repeated the same and now i see chunks of white matter coming out...what is this and should i be alarmed? what should i do? smell is gone and pain is gone since draining it...i ve been keep it clean and it is shrinking although there is hardened skin around the open area still...