My six year old daughter has an underarm lump, what can cause this?

Lymph node: This is most likely a lymph node that has been stimulated by bug bites, skin irritation or infection on the skin of the same arm.The lymph nodes are usually pea sized until stimulated & when doing extra work sifting through contaminated tissue serum they swell & sometimes become uncomfortable.Over a 2-3 week period they peak in size & then shrink.Rarely they become abscessed & need to be drained.