Some silkies have vaulted skulls, which is the "lump on their head" that you are seeing. I have three little silkie chicks with vaulted skulls that i got on saturday.
Those are chicks with vaulted skulls. They will be more easily injured because vaulted means they have a hole in their skull. Alot of people think that this produces bigger crests when they get older...I have no opinion on that. I've seen it both ways..