Lump under left nipple in toddler

[Crazylou]

Hi there little one 16 months old just recently got chicken pox while putting his cream on I felt a hard moveable lump under his left nipple, really concerned what it could be, wether it's hormone related has anyone experienced this before

I think I recall it being fairly normal in toddlers. Pretty sure DD had this too (not entirely clear on that though). It goes away.