What Causes Lump Under The Left Chest And Right Breast?

I was holding my baby last night and i felt a lump under his left chest behind his nipple.. it was about the size of a large grape..and seemed to move around if i pushed it... i was very nervous until i noticed he had another similar lump under his right breast, but that one wasn't as big... is this something i should be very concerned about? I read somewhere that it might just might be from the hormones being passed from his mother when she breast feeds... (he is 5 weeks old and has only been breast fed... although recently we have been mixing some rice cereal for supported sitters in with one of the bottles we give him before bed).. please let me know what you think, his next dr visit isn't until next week... thank you.