After 21 days on biaxin (clarithromycin) and a course of prednisone, sinus pressure still a problem along with severe headaches. Could this be med withdrawal?

No. Could be that the biaxin (clarithromycin) did not treat your sinus infection successfully. There are no studies i know of that support using Prednisone to treat sinusitis. It can decrease the effectiveness of your immune system. Consult your doctor.
Possible migraine . Migraine like headache can be caused by a bone spur in the nose. A ct scan of the sinuses is indicated and if the spur is present a septoplasty to remove it is indicated. I provide relief to almost 90% of patients with this problem. Visit my website for more information at www. Kevinsmithmd.Com.