Hi,
Usually any lump under the armpit (axilla) is concerning but not too worrisome considering your young age.
It could be something as simple as a cyst or inflamed hair follicle that may be caused due to shaving or use of antiperspirants (as opposed to deodorants).
Also a lump in the axilla area may occur when a lymph node is swollen due to bacterial or viral infections,and vaccinations,
Because of this possibility,you should see your physician for evaluation.
Take care...