I WOULD CALL THE PEDIATRICIAN IMMEDIATELY!
I am a cancer survivor (Hodgkins Lymphoma specifically). My largest mass was in my mediastinal area, inside my rib cage. To my knowledge, there are no topical lymph nodes in that area.
Lymph nodes do swell in response to infections. Usually, it's in the areas of the neck, under the armpits, the groin area.
95% of lymph node swellings are benign and have no reason for concern.
As someone who found a small, suspicious lump in my clavical area, I'd urge you to call the pediatrician and ask their advice. It may be hormonal-related. But, I would NOT wait a month in case it is something more serious.