Angular Cheilitis: This condition is characterized by inflammation of the corners of the mouth, which can lead to swelling, redness, and sometimes a yellowish discharge. It's often associated with fungal infections, vitamin deficiencies, or mechanical factors like drooling or licking the lips. The symptoms described, such as swelling lips and yellow-colored stuff where the lips touch at night, align closely with angular cheilitis, especially considering the absence of cold sores.

Eczematous Cheilitis: This is an inflammatory condition of the lips that can cause dryness, scaling, and swelling. It might be related to allergic reactions, irritants, or atopic dermatitis. The presence of swelling and sensitivity without cold sores could suggest eczematous cheilitis.
Impetigo: A contagious bacterial skin infection that can cause red sores on the face, which can burst and form light brown crusts. Although it typically doesn't present with yellow discharge between the lips, it could be considered if the yellow material is interpreted as a crust or if the infection is in an early stage.
Lip Licking Dermatitis: This condition is caused by chronic licking of the lips, leading to irritation, swelling, and sometimes a slight discharge. It's a possibility if the yellow discharge is related to saliva and skin irritation.

Squamous Cell Carcinoma: Although rare in young individuals, any persistent lesion or swelling on the lips should prompt consideration of oral cancer, especially if there's a history of risk factors like smoking or excessive sun exposure. The absence of a clear "lesion" makes this less likely, but it's crucial not to miss.
Syphilis (Secondary): This stage of syphilis can cause a wide range of symptoms, including mucocutaneous lesions. While less common, syphilis can cause lip swelling and should be considered, especially if there are other systemic symptoms or risk factors for sexually transmitted infections.

Gumma of Lip (Tertiary Syphilis): A rare manifestation of late syphilis, characterized by the formation of gummas (soft, non-cancerous growths) on the lips or in the mouth. This would be an unusual presentation but should be considered in the differential diagnosis of lip lesions, especially with a history suggestive of untreated syphilis.
Melkersson-Rosenthal Syndrome: A rare neurological disorder characterized by recurrent, often unilateral, facial paralysis or weakness, swelling of the face and lips (usually the upper lip), and the development of folds and furrows in the tongue (fissured tongue). It's a less likely diagnosis but could be considered if the swelling is part of a broader set of neurological symptoms.