Can you give us some more info?
How long have you had this fish?
How long has the bump been there and did it develop fast=a couple days or slow, a couple weeks?
Is the fish eating normally? Swimming normally?
What size tank, how many and what species of fish?
What are your water parameters?
In the meantime: do a partial water change of 30% using a good quality dechlorinator.
Bumps can be nothing to worry about or something that needs medication/treatment so let's get some more info and see if we can figure out what's going on.