Were you diagnosed with a sinus infection by a doctor? If the answer is yes, then you probably have an infection of the upper sinuses, which antibiotics often fail to reach, because of the lack of blood supply to deliver the antibiotics. Saline rinses don't work up there, because of gravity.
What may possibly be the case (although not for sure) is that the passageways to the upper sinuses gets swollen and nothing drains out, causing the pressure.
You can get balloon sinuplasty to open up the passages, or you can try a Sinus Flush. The saline from the flush will, usually, shrink the swollen tissue and open up the passages without surgery, and subsequent flushes will wash out the infected pus, and healing results.
http://www.medhelp.org/user_journals/show/2322
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground.
Good luck.