My baby (CAH) was on miralax for quite sometime.  I know it does give you bowel distress, gassiness, etc.  If memory serves me correctly the Miralax is a large white bottle that you measure out the dose in the cap.  I remember being told that up to the mark was an adult dose, so I was to give Christi a 1/4 or 1/2 (depending on the severity of the constipation).  (This was the dose for her under two years of age).  If your daughter has been complaining of stomach troubles I would assume that it is the Miralax putting her in distress.  As for the other medicine you mentioned, I have no experience with it.