Starting solids does change their stools, they will get thicker, can change colour and smell slightly. But if you think it is upsetting her system, I would be stopping and starting again at 6 months. They may seem ready to physically eat but their digestive system can still be to immature. Rice cereal can cause problems for some babies, ie constipation, tummy pains, so some bubs skip it all together.

I really suggest looking into Baby Led Solids. There is some great threads on BB about it.