Yes, a rise in creatinine would make her feel unwell, but I wouldnt have thought it would happen after just one tablet - was that the first tablet she had? CRF cats have good days and bad days, so maybe she's just having an off day, although it might be worthing phoning the vet to see what they say. Helen (Tiggy's Mum) might know more about the effects of Fortekor, coz her Tiggy's been on it for a while now.
As regards the food, I would just feed her what she'll eat, don't worry too much about amounts, and I wouldnt bother with the dry version - wet food is much better for CRF cats, they need all the moisture they can get. I give my Suzie additional liquid in her wet food. I also have bowls of water all over the house, so she never has to walk too far to get a drink!