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Every swelling is not cancer. It can be a fibroma. It occurs due to trauma or biting lips. The second one is a mucocele. It is a swelling of sebaceous glands. It also occurs due to injury or biting. Both are non-cancerous. Please send a picture of it. So that, I can guide you further.
Please say whether your mother has a habit of nail-biting. The management depends on the size and nature of the swelling. It may need some drugs or excision. You can consult a dentist and an oral medicine specialist soon for treating it. Please mention about her habits, drug history, and the presence of any sharp tooth in her mouth.