I both my hamster last night and everything was fine but this morning it has a lump or bump that seems to be hard on the side of his neck or cheek. Can a tumor grow in one night?
A lump can be anything, may be a Cyst, a Tumor, or Abscess. But if it appeared overnight, then most probably it is haematoma. Hematoma is a condition where blood starts to flow from the damaged vessels towards some particular injured point. That place swells due to this reason. Haematoma is a result of a fall, or a back knock. So take your hamster to a vet soon.