if stays for long then due to the infection there can be lethargy
but directly the sinus infection does not cause memory disturbance .
Until the sinus disease has spread ed and putting pressure on the brain.
If you have got done a CT scan
its no harm in consulting your doctor who has operated you.As they know better the disease you had and what surgery they have done.
If you are taking antihistaminic tablets for long time then also you may have lethargic feeling .