Could Tiny Movable Lump In The Armpit Be A Matter Of Concern?

My son has had a tiny moveable lump in his armpit for the last six months. When he noticed it we took him to the dr. She had a hard time finding it because it was so small, and she said it was a nonissue. Tonight he pointed out that it seems bigger. I felt it and it still seems slightly smaller than a pea. Should we be concerned?