Why is there a large painful lump in my armpit that has been growing over the past couple of days? how do i get rid of it? it hurts to move my arms?

That location has lymph nodes, odor producing glands and sweat glands, all of which could be invaded by germs. Germ invasion can produce a swollen lump/cyst. Moist heat may increase blood flow enough that your natural healing factors could help, but sometimes these need antibiotics or even surgical drainage before they are cured.

Please let your parents know so that you can see your pediatrician for an exam and proper treatment.