My first guess would be you are over doing it on either the water or the food. I didn't have an over fertilizing problem myself. But I did have over watering issues. My plants started to turn yellow in the first few weeks after I set them out. My problem was over watering and under fertilizing. Tobacco wants moisture all the time but not a lot of it. Indoors in potting soil and it might not need much feeding at all. At least at first.
My second guess since you have them indoors is root bound. I will say if you get it figured out and fixed the plants will recover and keep on growing.