What Causes Recurring Cyst Formation Underneath The Armpit?

Every once in a while I'll develop a cyst underneath my arm pit where my bra strap wraps around. Usually I can pop it and it goes away. Last night, I saw one that had a head on it so I tried to pop it. The puss was minimal, very slow to come out, and hard. As I applied more pressure I felt the familiar "pop" between my fingers but nothing came out. It immediately tripled in size, became much harder, and turned a very deep purple. I think I popped it but it popped underneath my skin. Today it is still large in size, hard, and also tender and pink. I'm not sure if I should try to pop it again or leave it alone. I'm no stranger to popping cysts and have never had one that popped like this and stayed underneath my skin. I appreciate any advice you may have. Thank you!