I have a newborn male baby with birth weight 3.5kgs. He is 4 weeks now has nasal congestion wght 4.8kgs. Breastfeeding. Is it bcoz of what I ate?

More likely irrritan. Nasal congestion at that age is most often due to exposure to fragrance, dust, smoke or other odors.This often causes swelling inside the nose & mucous buildup. Saline nose wash used in liberal amounts will often clear this up.
Stuffy nose syndrome. It has nothing to do with what you eat. Babies breath through their noses and get a lot of mucous in their nose. If the room in which they sleep, is too hot or very dry the problem can get worse. Use the bulb syringe that was given to you at birth and as suggested saline nose drops.A room temp of 68 degrees is good and humidity should be between25-30 percent.