A common cause of a swollen lower lip in a young cat is a condition called eosinophilic granuloma. It can have many different underlying causes, and is typically treated with corticosteroids. Here is a link to a page about this condition:
If the chin is also swollen and has black crusty material on it, it could be feline acne. Here is a link to a page about this problem:
My recommendation is to take your kitty to the veterinarian for diagnosis and treatment.
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