What Causes Lump On The Gums Of An Infant?

hi my 2 month old has a lump on his lower right gum its hard and doesn t seem to bother him at all but i am concerned. I didn t notice it until 5 days ago and i took him to his pediatrician 2 days ago she looked at it but says she had never seen anything like that before she says it could be nothing. What do you think?