I feel a rubbery bump under skin on back of neck - could it be cancer?

Bump. Cysts, lymph nodes, skin cancers, etc may all appear as bumps under the skin. See a dermatologist for evaluation.
Probably Lipoma. Cancers are very rare under the skin. The most common rubbery lumps are benign growths of fat tissue called lipomas. They can become quite large, especially at the base of the neck and upper back.