Sinus pain with no congestion only at night. what could be the cause?

Depends: You might have sinusitis. If an isolated ethmoid sinus or frontal-sinus or sphenoid sinus becomes obstructed or infected it can cause pain even with no other sx. Clues are one sided pain, worse when you lean head down low, pain that responds to antibiotics. Other mechanical problems can cause this. Go see a Doctor.

Could be infection.: Sinus infection can occur without congestion. This symptom could also be a dental issue. Follow up with PCP, and perhaps dentist.