My DD was similar. I find she would take water from a Calpol syringe and diluted milk in a bottle to increase the Oz's.
Best to call your Health visitor for recommendations.
From memory I was told to try: * juice of half an orange diluted with water (1:1 I think) to help get the bowls moving * if this does not help, give very small amount of prune juice but best to check first * mixing more water into food, giving yogurts and custard type deserts for example. * mixing plenty of milk with baby rice / cereal in advance of feeding time because this absorbs a lot of liquid.
I would expect fruit and veg to be the best food types, especially those with high water content. With meat, dried cereals and harder to digest ingredients the worst.
It's the water I think. When my dd doesn't drink enough she has a very bad belly. Try loads of different cups and bottles until you find one she likes even a few sips a day will make a world of difference