My eldest son had a very similar thing when he was 3. He would be fine for 20 minutes then in agony. We took him to emergency and he was admitted for observation and given x-rays which showed that he was very backed up with poo so they said he was constipated, but he was refusing to poo (being only 3) because of the pain so in our opinion the pain caused the constipation, not the other way around. Nevertheless we gave him laxatives and then a few days later (of almost constant diarrhoea because of the laxatives) we took him to the GP who said the pain was most likely colic, basically causing spasms of his stomach, and that this was most likely caused by stress. At the time we had just emigrated from Australia to Scotland and he was very unsettled. Once he was more settled it all stopped.

Anyway my advice would be to take her back and hopefully get x-rays to see if there is a blockage, but also consider if she may be stressed and try and deal with the psychological side of things too. Good luck, I hope she feels better soon.