My 4 year old has a small movable (pea sized) lump under her armpit. When I first noticed it about a month ago, her doctor said there was nothing to worry about.  It is still here and it feels a little bigger and she now says it is sore.  What could this be?
Hiding worry. Most common cause of movable lump under arm pit is lymph node, swelling due to hair follicle infection called fur uncle. There will be visible swelling with redness and pain if its due to auricle. It is usually of short duration. I feel it is lymph node as you are saying of 1 month duration.don't worry most of the times lymph node is inflamed. That maybe she complaining.still in doubt & no visible swelling advised high frequency scan to underlying cause. Thank Vasundhara