I have been on antibiotics for a sinus infection for almost 3 days and not feeling any better. why is this?

Check it out: I usally ask patients to give an antibiotic at least 48-72 hours. If no improvement noted may want to check with your doc.

Possibly....: It is possible you have a viral infection. If that is the case, it should run it's course over the next 7 days or so. If your symptoms are getting worse, then see your doctor again. You may need to se an ENT doctor for a full evaluation, especially if your symptoms are worsening.