Large lump under right armpit

I have a very large lump under my right armpit. It is very visible, and is extremely painful to touch.

It seems to be really big and painful in the mornings, and gets a little smaller as the day goes on (but still sensitive).
My armpits have been sensitive for about two weeks, feeling like a bruise when i would touch them. The large lump only appeared about three days ago.

I went to the doctor and she said that it is definitely a swollen lymph node, and if it doesn't go away within another week to come back and do blood work.

What does this mean? What could cause this swollen gland, and what is wrong with me?