You can have that nasty gunk up there and not feel it. There are people in the hospital all the time who get CT scans for other issues and are told they have sinusitis. In a recent study, almost 40% of people with a chronic cough had sinsusitis show up in a CT scan. Yet they had no nasal or sinus symptoms.
The oil pulling didn't cause the mucus, it's been up there the whole time. What it did was trigger your sinuses to start cleansing themselves.