If your UC isn't a mess and this is a run/ walk program, you're probably not overdoing it. Sounds like a side stitch. Try a forceful exhale from your belly; you might even make some noise while you're doing it.
Otherwise, it might be your abs or core that's just not used to the exercise. You might incorporate some crunches on the off days. You don't have to overdo it. A dozen or so every other day to start will help.