Your either eating too much food or eating to close to activity, AIS guidelines are a meal 3-4 hours prior to activity or smaller portions closer to activity. SO an early morning ride, a decent breakfast is not optimal inless you like eating at 4am.
The issue is is you fill your gut up, gastic emptying slows down and the food sits in there longer - inregards to a stitch, I have ever seen anything conclusive to what a stitch is, but it may be a ligament like structures that hold the stomach in position getting stretched under the load of a full stomach during activity. I've dissected a few cadavers and not been able to find this elusive structure that may cause a stitch. But it certainly only seems to occur with a full or near full (fluid or solids) stomach.
AIS guidelines - very good evidence and practical experience.
http://www.ausport.gov.au/ais/nutrition/factsheets/competition_and_training/eating_before_exercise