What time of the day do you give her the cereals?
I started giving cereals around 5 months. At first I was giving it to DD at lunchtime, and have now moved on to breakfast in preparation of giving her veggies at lunch. When she wakes up I give her a bottle, then we play for about 20-30 minutes, then I offer her the cereal. After that I offer her the bottle again, to which she'll only drink one oz, but after we play some more and I prepare her for her nap she'll drink the rest of the bottle. She may be drinking a little less formula for her morning bottle, but only about 1 oz less, so no big deal. I don't know if it's because she's active during the day that it makes her hungry enough to have formula and solids.