Popped a lump in the middle Armpit area

Patient: Hi there, ive just found a lump under my armpit and decided to pop it. abit of puss came out(white) and then afterwards somekind of very little tiny sac came out. it stings when i touch it and on close up it has blood, although the sac is grey-ish. What could this be? What do i need to do?

Symptoms: Stinging armpit