Dear Laszlo Jamf,
Since you are noticing a number of sensations in your body, not just the pressure in your sinuses, I'm fairly confident that this is a side effect of stress release.
Stresses can come from painful experiences in the body, are stored throughout the body
http://www.nsrusa.org/about-stress.php) and, when released, can generate sensations in the body. When the release of such a stress completes, the associated sensation disappears.
Usually, stress release happens only during meditation. But if the stress is a big one, some release can continue during activity. There are ways (discussed in other forum postings) to eliminate such unwanted stress release, or to decrease the discomfort they can cause.
Since sensations are indicators that stress is actually being released, they are evidence that something very good is happening. The elimination of stress is helpful to life in many ways.
It actually doesn't matter if we have sensations in the body or not. As long as we are transcending thought effortlessly twice a day, we are dissolving stress and will eventually notice benefits in our daily life, including deep refreshment, increased energy, and improved intelligence.
Just for completeness, it is also possible that you have a medical problem, such as a sinus infection. If the sensation continues for more than a week or two, you might consider having a doctor check it out. See also
http://findarticles.com/p/articles/mi_m0NAH/is_3_33/ai_104836631/ for some helpful hints.
David Spector
NSR Meditation/USA