Runner's Training Tips: belly is achin'

sometimes my midsection/belly aches after I run.  Mostly it's the upper part of my abs.  could this be poor core stability or something more?  and any recommendations to help it out.

Is it muscular?  If yes, add some core exercises to your routine - Planks, stability ball sit ups, deadlifts - runnersworld.com has some great general routines that will get you in and out of the gym quickly - and most of it probably doesn't require a gym to boot. Definately toward the end of a long run, i can feel if I'm not taking care of business with the core.