Having Nasal Congestion With Clear Watery Discharge, Occasional Sneezing, Some Post Nasal Drip With A Slight Dry Cough, Mild Headache. Taking Omnicef. Cause?

Here are my symptoms: Nasal congestion with clear watery discharge, occasional sneezing , some post nasal drip with a slight dry cough, mild headache mostly on the left side, eye tearing in both eyes but worse on the left, mild pain on the left side of my head when laying on my left side, mild pain above above eyebrow and in the cheek and in front of my left ear. I recently completed 10 days of omnicef for what the doctor thought was a sinus infection. While my symptoms did improve, they have returned a few days after finishing the antibiotic. I am very concerned that this is not a sinus