Son Is 4 Yrs Old, Bumped Head* (says He Did

Son is 4 yrs old, bumped head* (says he did -didnt see it happen) , no egg showed until day later, now has large egg on corner of eye , near occipital bone, no bruising just red color and about 1/2 thick. Should we bring to e.r.?- had similar reaction with his mom when got a horsefly bite..