hello,I have got your question.your mother's sugar level is within normal limit as you said.So this type of headache may be due to tension headache
or may be a migraine
,as you said is normal.is any other symptoms present with the headache? is she on insulin/other drug I don't know.So do a thorough check up by your doctor first.and follow what he suggests. wish your mother a good health.thank you.