The leading cause of cramping is dehydration
You need to be drinking one 750 ml (3 cup) bottle of water and/or gatorade every 1 to 1.5 hours while you ride.
If you are racing, you should have been drinking about that much every 1 to 1.5 hours in the few hours leading up to the race. Stop drinking at least half an hour before you race though.
The next cause is lack of electrolytes. Because it is fairly warm where you are, you might consider eating a small bag of potato chips or salted almonds an hour or so before the ride. Do drink some gatorade, and the banana was a good idea.
Lack of some of the other minerals such as calcium and magnesium can also be a problem. Go for a TUMS or Rolaid half an hour or an hour before the race.
Also, don't forget to stretch. When you get to the race, go for your warmup ride, then stretch a bit, and then do the race.
All the best!