Discussions that mention potassium

Exercise & Fitness board

It might just be a cramp. Make sure you're hydrated before you run and make sure you're getting enough potassium. Also, make sure you're not tightening your abs too much when you run because it can create an irregular breathing pattern and can start cramps in the guts. And make sure you get a chance to "clear the pipes" before you run so your guts aren't working against you. A cup of tea or coffee can usually help.
If none of that helps, you may have something more serious going on and might want to mention it to your doc.