i have major sinus problems as well, had sinus surgery last year. I started oil pulling and stopped the dairy at the same time. My sinus's have been pretty great since then (about a month, maybe a little bit more). But one day at work we had pizza for a treat with lots of cheese and i indulged. Next day had a really bad sinus headache that took about 2-3 days to clear up. I would say your sinus's are trying to rid themselves of the mucas you built back up into them. I don't think oping can do that alone with a poor diet if you are prone to sinusitus. Are you irrigating your sinus's? I irrigate mine with a waterpik for pulsating irrigation. You have little hairs in your sinus's called cilia that pulsate like your heartbeat to push the mucus out of your sinus's. When you have an infection or just too much mucus in there, the cilia stop pulsating. The waterpik will jumpstart the cilia back into it's pulsating motion again, while also flushing the existing mucus out of your sinus's. I use a sinus irrigator attached tip, it's the best. Whenever i have a sinus headache i irrigate and op and usually i can knock it out.
ps. this is a link to the irrigator tip although mine is just the blue tip part that you can slip right over your existing waterpik