I have a similiar problem. I used to love yogurt with homemade granola as my afternoon snack, but now I know I can't eat that on run days (I run after work). It actually gives me horrible heartburn, of all things.

Lately, I've been happy with an apple and 1-2 oz of low fat cheddar. Just a plain apple would probably leave me with stomach distress, too. For whatever reason, the addition of the cheddar keeps things on an even keel (protien? fat? who knows...)

I agree with Oak - the best way to figure it out is to experiment.