Sounds like a side stitch. You might want to leave a little more time between eating/drinking and running. I typically stop drinking any liquids about a half hour before running. If you're drinking water regularly throughout the day, you're in no risk of dehydration until you get out to 45+ minutes of physical activity, so it isn't a big deal not to drink gallons of water right up until you start like many people think you need - and in fact, that can cause side stitches and stomach cramps.
Remember to breathe deeply when running. You want to take your diaphragm through its full range of motion. If you get a side stitch, all you can do is slow down, and take deep, "belly" breaths.