Are you offering water/EBM/a quick BF with/after solids? That was something I didn't do with DS til we had similar issues.
Also, how are her poos? The added iron in the cereal can constipate LO's. It is quite possible she's just getting used to how it feels to have solids moving through her - she's only had milk til now. Remember how she acted when she was newborn and feeling milk go through her tummy and doing a poo? She may have squirmed, gone red in the face, or even cried - this is related to the new sensation.
For now, I'd just BF for a few days, do some tummy massage and see if she comes good. Solids introduction is a slow process. I stopped for a week, then gave solids once every two days when I started again. TBH, I never did rice cereal again and we never had those issues again, though we often have issues of some kind if we use bought jars of food.
re: naps/night sleep - she may not be sleeping as well and therefore not as rested, so she's getting OT from the same A time she can usually do.