There are several different techniques for helping with this issue. What worked best for me was slowing down my pace slightly and breathing very deeply through my nose and forcefully exhaling through my mouth until the stich went away from my side. Your ribs relax downwards after having a baby, but side stitches are very common in running. Most of the time it boils down to your breathing technique and pace. Here are some suggestions I found online: http://www.ehow.com/how_2047713_get-rid-side-stitch-running.html
As with many things in sport, side-stitch cramps have led to an extensive mythology regarding their origin, almost as storied as hiccups. The e-how explanation above is very helpful in that it echoes the more recent explanation summarized below by popular physician Gabe Mirkin, MD, that goes into a bit more detail. However, e-how appears to morph many of the popular explanations into the scientific one originally formulated by Dr. Tim Noakes (Lore of Running), which makes more sense taken straight-up, as offered by Mirkin.
Dr. Mirkin's expertise is drawn not only from his many years as of board-certification in Sports Medicine and Pediatrics, but from running over 40 marathons himself. Noakes is a professor of Sports Medicine at the University of Cape Town with over 70 marathons to his credit.
The explanation, in a nutshell, is that ligaments holding the liver in place are stretched more on the right footstrike during exhalation. It has been suggested that one way to minimize the cramping is to begin exhalation on the left footstrike. A way to work out the cramp with direct massage is also proposed.
This is an interesting injury (unless you are the one with the injury). I posted a case study about this on my website about a month ago. You can view it here.http://injuredrunner.com/injury-cases/Rib_pain_pregnancy.htm
As to the origins of side aches that was discussed by James Johnson there is actually a researcher from Australia that has published 8 of the 13 research based articles on side aches. His name is DP Norton. He set out to test the hypothesis set forth regarding the origin of sideaches. His conclusion is that sideaches are the result of friction in the parietal peritoneum, which is the inner lining of the abdomen. Interestingly, he disagreed with the theory that the ligament supporting the liver was involved in side aches. He bases this partially on his finding that swimmers had a high incidence of side aches and yet they should not have the jostling that is postulated to aggravate the ligament that supports the liver. Others in sportsmedicine have postulated that side aches are a muscle spasm of the diaphram. He also disagrees with this theory because horse back riders have a relatively high incidence of side aches but they do not breathe vigoruosly. He also found that breathing volume did not change during a side ache which he felt was further reason to disagree with the muscle spasm theory.
I know this doesnt directly address your issue, but I hope you find the case study helpful.