This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
If you had your baby less than 6 weeks ago, it is likely that your uterus
hasn't returned to its normal size, and this could cause the cramps.
Remember, too, that your stomach muscles separated during pregnancy and it
takes time for them to meld together again.
The important thing to remember when returning to running after a layoff is
to ease back into running, paying scrupulous attention to how it feels. The
old adage, "listen to your body," applies here. If your stomach is
cramping, slow down, ease up.