There are a few possibilities -
The antibiotic might not have been effective - anaerobic infections
often occur in sinus.
There may be an underlying cause - a polyp for example
Rarely the headache could have a different and unrelated reason!
You may need an X-ray (may be a CT scan
) of the sinuses
- please see an ENT specialist
and have you evaluated
In the meantime, use decongestant nose drops - available over the counter