Painful lump under armpit

I'm 16, female and have found a pea sized lump under my arm pit that i have always been quite cautious of because i thought it was a brown mole and it has been there ever since i started shaving my under arms from about age of 12.
i dont know if i have adjutated it by shaving it, but it used to look healthy and it even had a hair growing out of it. now its a raised sore red lump, even the area around it it painful. it used to have a small amount of puss come out of it which i thought may have meant it was a spot. but after squeezing i soon realised it wasnt. i dont think its an in growing hair either.

ive recently got really worried because when i move my arm sometimes i get a shooting pain and it feels like its coming directly from the lump. im desperate for some advice and im worried it might be cancer, but im too young for that?