What Causes A Lump In Right Upper Arm Post A Bout Of Flu?

My child was diagnosed today with the type B flu. We just noticed a lump on the outside upper right arm about the size of an egg. Feels deep and not on the outer skin. Can this be due to the infection from the flu and/or medication prescribed. The lump has not been there before today. Should I be concerned and or take him to the ER? Thanks in advance, Chris