I don't know if that's possible, so I asked a Le Leche League leader, the Run Like a Mother group, the Runner's World forum, and the Twin Cities API group.
Here's the info I got:
- Kellymom says that if you're exercising at max, you can get a build up of lactic acid, but it doesn't usually bother the baby.
- The responses I got from the Runner's World Forum all said it was probably a bug.
- The Twin Cities API list said it was probably viral.
- The Run Like a Mother group had never heard of anything like that and referred me to the Kellymom link.