Running/side cramp mystery. Can you elucidate?

I’ve been running for 3 years now and something I’ve noticed is a mystery to me. If I run on an empty stomach I get side cramps. If I run after eating toast/cereal I get side cramps. But if I run after eating oatmeal I don’t get side cramps. In the eating scenarios I wait the same amount of time (usually about an hour to an hour and half) before running.

Why does eating oatmeal matter?

I’m glad oatmeal seems to cure the cramps because side cramps suck.