The stomach is the major consumer of blood outside of exercise because it takes a lot of nutrients and energy to digest food believe it or not. And then obviously it has to cart away what it gets out.
When you start exercising the body adjusts to allow your muscles the blood they need, this is why the heart pumps both faster (more beats for minute) and more per beat (greater stroke volume). It should even out and the stomach should still get about as much of the blood that it did before exercise but while the body is adjusting it doesn't always get enough and it's believed this is the cause of some of the stomach pains you may feel when you exercise
So it can be simple way of your body telling you the stomach isn't getting what it needs. If you know this then you can figure out for yourself if it's serious or something that'll figure itself out.