TAKE BABY TO HOSPITAL? Please help!

My son is five months old, today I noticed a soft bulge on the left side of his head it's like someone stuck soft dough in a giant goose egg on his head! What do I do?! Should I just monitor it? I'm scared that I'm over reacting and gonna already use one of the three hospital visit he has for the year, what do I do?! Cause what if it's water on the brain or a concussion?