Acute Sinusitis: This is the most likely diagnosis given the symptoms of sinus congestion, cold-like symptoms, and mild tenderness in front of the sinuses. The fact that DayQuil was unable to alleviate congestion suggests a possible infection that requires antibiotic treatment.

Allergic Rhinitis: Initially, the patient thought it was allergies, and the symptoms could still be consistent with allergic rhinitis, especially if the patient has a history of allergies.
Viral Upper Respiratory Infection (URTI): The progression from feeling like allergies to cold-like symptoms could indicate a viral URTI, which often presents with similar symptoms.
Chronic Sinusitis: If the patient has a history of recurrent sinus infections or chronic nasal congestion, chronic sinusitis could be a consideration, especially if symptoms persist beyond the expected duration for acute sinusitis.

Invasive Fungal Sinusitis: Although rare, this condition can be deadly if missed, especially in immunocompromised patients. Symptoms can be similar to those of acute bacterial sinusitis but may progress more rapidly.
Cavernous Sinus Thrombosis: A potentially life-threatening condition that can arise from sinus infections. Symptoms might include severe headache, fever, and neurological deficits.
Meningitis: In rare cases, sinus infections can lead to meningitis, an infection of the protective membranes covering the brain and spinal cord. Symptoms can include severe headache, fever, and stiff neck.

Wegener's Granulomatosis (Granulomatosis with Polyangiitis): A rare autoimmune disorder that can cause sinusitis among other symptoms. It's characterized by inflammation of the blood vessels and can affect various organs.
Tumor or Cyst in the Sinus: Although rare, a tumor or cyst in the sinus could cause chronic or recurrent sinus congestion and should be considered if symptoms are persistent or atypical.