I think it really depends on what the bump looks and feels like, whether it is soft, hard, moveable, or fixed, and how long it has been there. Some babies do get small bumps on the head that turn out to be nothing, but the fontanelle area is something doctors prefer to check in person rather than guess over a message. I would bring your baby to the PD soon, just to be safe, since they can feel it properly and tell you right away if it needs attention or not.