There are mucus producing cells in the lining of your sinuses (and the rest of your airway). Those use the water and other ingredients that your body has on hand (from being well fed and well hydrated) to make mucus, which then clogs up your sinuses because they're making a lot of it,. It's also thicker than usual because it is trapping all the infection, and because probably the lining of your sinuses is swollen from the infection, which makes the drain holes smaller, so the mucus doesn't drain as easily.