Hello sir,
Thank you for posting your question here.
Diet cola contains around 4.85 mg caffeine per ounce of the drink,so when you consumed two 20 oz drinks you comsumed around 194 mg of caffeine.The daily limit for adults is set a 400mg and for children its much lower.The effects of caffeine depends on so many factors like your age,general condition,caffeine sensitivity(differs from person to person),other drugs you are taking etc.
So even if 194mg is on the safe side.So if you are sensitive to caffeine, and take drugs that are contraindicated with it you may feel some adverse effects even at lower doses.
The adverse effects include difficulty to sleep, reatlessness,irritability,
nausea,
vomiting etc.
Lightheadedness is commonly caused by a fall in blood pressure or
heart rate,caffeine has the opposite effect.So your lightheadedness may or may not be due to caffeine.
The effects of caffeine can last upto 8 to 12 hrs,so if you expirience the lightheadeness even after that,consult a doctor.
I hope i have helped.
Thank you.