Bruises or swollen.
wart
A bump is a lump like, for example, a bump on your skin or a bump in a road.
keloid...
wart
Wart
wart ormilla
Cyst
lipoma
A painful bump on your skin, usually worse than a bruise.
it may be a serois medical conditiion, ask a vet. is the rat old? is the bump big? is the lump swollen? is the bump on a patch of thin skin? where has your rat been? is the bump smooth or scaly or something else?
A hard bump under skin on the side of the nose that comes and goes sounds like a common zit. However, it also could be a calcium deposit.
The skin on the top of your head might be sinking in due to a bump you may have gotten to the skull. The skin will follow the contour of the skull.
that is because the grease of the bump grows on the skin and develops on it to create more bumps . At least i think that is true.