I don't know about augmentin, but amoxicillin has skin rashes listed as a side effect.
Add a little xylitol to the saline solution - it's a natural antibiotic
Stop the medication. And go to the FDA site and file an adverse reaction form. We don't know for certain that the antibiotic is linked to the red rashes, however if an allergy has developed there is a remote but unlikely possibility that the allergenic response may worsen and become life-threatening. You have already been on the antibiotic two weeks, which is long enough (under normal circumstances) to clear an infection. Sinus infections are notoriously difficult to destroy. The organisms become encapsulated in biofilms and antibiotics will not touch them. Long terms of antibiotics are often prescribed...far longer than the normal ten days. And if bacterial in nature they may be resistant to a broad spectrum antibiotic. In general the drill is multiple flushing of the sinus passageway with normal saline several times a day, right and left nostril. It is best to consult a specialist when you have a long-standing sinus infection.
Finish the medication. Report area to the doctor.