Thank you for your query at DoctorSpring.com
I understand your concern.
No, the blood glucose definitely does not vary in seconds.
If there is an interval of 20-30 minutes it can alter, but not in seconds.
What was the gap between the values of 115 and 90?
It is better to get one single FBS/PPBS evaluated and more importantly HBA1c level checked, which will predict the control over the past 3 months.
Feel free to discuss further,