Hard lump under left armpit. Should I be worried?

Hi everyone,

 

back in January I noticed a hard lump under my left armpit that was not painful. I called the doctors and they said not to worry.

 

I sort of forgot about it and today my partner felt it and said it was bigger. It still doesn't hurt and like I said is hard to get touch.

 

my partner has told me not to worry and that it's probably nothing. What do I do?

 

I am 22 and as far as I know my family have no history of cancer.

 

many thanks