When my turtle lowers its head, a hard lump bulges under its neck; when laid flat it disappears. Is this normal, and why?

A hard, fairly large lump under a turtle’s neck may be caused by inflammatory material, tissue hyperplasia, or a tumor. We recommend taking it to a vet promptly for a microscopic examination along with a stained-slide reading. If tumor cells or other abnormal gland shapes are seen, the lump is likely accumulated inflammatory material and can be judged as infection.

Otherwise, it may be a hard lump from a tumor or hyperplasia that requires surgical treatment.