Hello. Welcome to hcm.
Headache after surgery is due to CSF leak which happens when anesthesia
is given at back. If the headche had started on 2nd day with decreasing by sleeping with out a pillow,it is most commonly leak. You need not worry and continue taking rest and sleeping with out pillow.
Rarely it can be a side effect of drugs like antibiotics etc.
If the pain doesnt come down in a week,then you need a consultation with your doctor to rule out others causes and treated accordingly.
All the best