Mucosal Hematoma or Hemangioma: Given the history of trauma and the description of a fluid-filled bump on the lip, a mucosal hematoma or a traumatic hemangioma (which could form as a result of the injury) is the most plausible diagnosis. The absence of fluctuance and induration supports this, as these features are more commonly associated with abscesses.

Lip Hematoma: Direct trauma to the lip can cause blood to accumulate in the soft tissues, leading to a hematoma. The fact that it's fluid-filled and not fluctuant is consistent with this diagnosis.
Traumatic Mucocele: A mucocele can form if the trauma causes minor salivary gland duct damage, leading to mucin accumulation. The description of a fluid-filled bump without fluctuance or induration could fit this diagnosis.

Abscess: Although less likely due to the absence of fluctuance and induration, an abscess must be considered, especially if there's any sign of infection. Missing an abscess could lead to serious complications, including spread of infection.
Infected Hematoma: Similar to an abscess, if the hematoma becomes infected, it could lead to severe consequences, including sepsis. Early recognition and treatment are crucial.

Lip Angioedema: This is a rare condition that could present with swelling, but it's usually more diffuse and can be associated with systemic symptoms or other signs of allergic reaction.
Lip Kaposi's Sarcoma: In immunocompromised patients, Kaposi's sarcoma could present as a lesion on the lip, but it would be unusual for it to be described as a fluid-filled bump following trauma.
Salivary Gland Tumor: Although extremely rare in this context, any persistent or growing lesion on the lip should prompt consideration of a tumor, especially if it doesn't resolve with conservative management.