lump under armpit?

So, I've had this lump for almost 2 months and I thought it was like... you know, a pimple. Like I would pop it and it would go away in a week or 2 but not this time around. I've popped, I thought I popped it all but NOPE! The lump has been there since 2 months. What is it? Could it be serious? Should I worry? Should I see a doctor?