well this is the way I understand it....Your blood can cover two of three area's at a time...
1. Your brain.
2. Your stomach
3. your Body for exercise.
Ok, your Heart will constantly feed your brain...So that one is consistant,
But your body Diverts blood to the stomach when you eat food, to help with processing, And then when your exercising it will divert blood from other area's to those muscles being exercised.
As the old wives tale goes, wait 20 min to exercise/swim because your stomach will cramp up. IE your body does not have the Blood it needs to supply all area's at the same time, The exercise will take over for the stomach if you exercise right after eating.
Not sure if it is true, but I makes sense to wait at least a half hour after eating if you are going to exercise. I would not exercise on a empty stomach