This happens to DD too. We have had a couple of times where she has been in real discomfort and had some explosive movements..... Doc took a sample and nothing came up. But almost everytime she goes swimming we hear 'I've got a sore tummy' followed by a bout of the runs. We got to the stage where we couldn't make any plans on swimming night (a Friday) because we needed to be near a toilet. DD is an awesome little swimmer and doesn't swallow any water. It happens at different pools too - at least 4 of them so it's not a pool specific issue.

She's started swimming again after the winter and although the sore tummy is still there, I don't think we've had the runs. She's 6 now.