Hi welcome to Health care magic forum. Your mother is feeling giddy because her blood sugar raised above 300, and it should immediately get it to normal. That is why doctor asked you to give insulin
, other wise the vital organs like brain, will not have enough of glucose
for maintenance of health of those organs , and that is the reason for the reeling sensation.
What is missing is whether she is using insulin from the begening, or now only doctor advised to give insulin. If she is getting it from the begening, the dose should be increased and supervision is a must how is the blood sugar reading. If the reading is still high, dose is to be increased, and observed, if sugar goes below 100, it is dangerous so a close follow up is necessory.
If after getting the sugar to normal, also if there is reeling sensation, other causes like middle ear infection
or brain changes may be considered.
Wishing for a quick recovery and thanks for calling.