Thanks for posting your query.
Please take her back to the hospital and get her blood pressure checked. In addition, a check on the medicines that she is taking now is required. Headache is a common side effect of many drugs. Also, if there was a spinal anesthesia
used for the surgery, it is bound to cause Hypotension
and headache in the post operative days.
Please make her drink more fluids.
I wish her quick recovery.