No sign of a present infection but after getting over a sinus infec. i'm still having intense sinus pain left side of face. cause?

Blockage/discharge: If you have intense pain, that sinus might be blocked and depending on the type of anti-biotic and the course of treatment, the sinus infection might have not been fully treated. I suggest doing saline rinses (netti pot or ocean spray), warm compresses and decongestants like Mucinex (guaifenesin) d and if the pain persists, go see your doctor for evaluation.