Just to confirm what the previous poster mentions that sinus blockage can indeed cause lightheadedness (suffer from this myself) and may be resolved by the use of decongestants.
If you are on BP medication, make sure you speak to GP/Pharmacist first, as some decongestants can have an effect on your BP, if my memory serves me correctly particularly things like Sudafed (pseudoephedrine) can be a problem.
Edit - apologies for double information, gingernut had a simultaneous thought...
