Hard lump on head

My baby girl has a hard lump on the back left of her head....i brought it up to pediatrician and they couldnt tell me for sure what it was but didnt seem too concerned. It doesnt cause her pain. Roughly size of a marble... But i think it may be getting bigger..but then again she is growing. Of course when u google you get worst possible diagnosis. Just wondering if anyone else's little has this?