Both my girls have had this with two different issues. Dd1 was constipated but we took a few months to work it out as she was going to the toilet. She is on osmolax now and her tummy is no longer sore and her toileting accidents are improving.

Dd2 complained of a sore tummy one week and was throwing up the next. Took her off dairy and she stopped throwing up but her tummy was sore. We realised by accident when dh took them that it was her vitamins upsetting her tummy and in turn she couldn't cope with dairy or rich foods. She has improved now she no longer has the vitamins you chew and has been starting to have dairy again.