Should i worry about this being lymphoma?

I'm a 15 year old girl and about 5 months ago i noticed a lump in my armpit, about the size of a pea. It's hard and seems fairly movable but totally painless. I completely forgot about it until recently and it seems noticeably bigger. From just looking at my armpit you wouldnt be able to see the lump but its is still here after so many months, should i be worried about lympoma of could it be something completely harmless?