Nutrition, calories, avoiding hypoglycemia and migraines

I had a low blood sugar induced migraine last week after a typical master's workout. I've been swimming with the master's (in the fast lane) for almost a year, and I have not had any issues with migraines. I am 32, have had 5 migraines in the past 8 years, and all of them can be associated with exercise, and I am thinking nutrition. I followed up with a doctor's visit, and he said, "yep, this is a classic migraine." Long story, I am freaked out. I am off to practice in one hour (during my work lunch hour) and I am nervous about having a migraine again. I know it is ridiculous, but I am not sure what caused it, other than perhaps not getting enough food in me prior to the workout. I want to be able to swim open water distance events from 2 miles to 6, but this "set back" has me battling mental games now. Questions: How many calories should I be consuming a day if I swim 2-4 times a week, 2,800 - 3,600 yards each time? Sometimes I run on off days. What should I be eating/drinking prior to my workout to avoid sugar lows and maintain good health? What should I be eating/drinking during my workout? What should I be eating/drinking after my workout? I have less than one hour until I am in the pool today, and I think I will start with chocolate milk and a PB&J sandwich, gatorade during the workout, banana immediately after, then lunch. Thoughts? Thanks!