General Fitness & Nutrition: abdominal pain

Hi,

I am fairley new to running, well to all physical excercise really, and just thought maybe could pick the brains of all you seasoned fit people! I have just lost 5 and a half stone (77lbs) though I still have a good few more stones to loose I have been excercising for the past 5 months in an effort to get fit and slimmer not just slimmer.

I am getting fitter and today managed to run/jog just over 3k for the first time without having to stop. However I had quite bad stomach ache for about 20 min after I finished. I have noticed when I do really push myself and work quite hard this happens and its actually really painful!

I am generally healthy although still overweight but am having investigations into gall stones, but the pain from those is different to the pain after excercising and its related to if I ate fatty food.

Is it because I am pushing too hard? Or is it common as the body is focusing on other stuff?

Thanks for your comments.

Julia