After agreeing with 'ask the doctor', I'd also suggest perhaps stopping the rice+formula and seeing if that fixes things. Adding cereal to formula is almost entirely pointless; if you're trying to get into solid foods, go another route - 6 months for most babies is old enough to eat regular food (ie, carrots or whatnot). The rice cereal in the bottle (I assume that's how you're giving it given the description) may be interfering with her eating her formula, and thus getting properly hydrated.
The old recommendations to start off with cereal in the bottles are very outdated, and even the recommendations to start with cereal at all are largely outdated - our pediatrician for both of ours recommended starting with vegetables, not cereal, and strongly discouraged cereal in bottles (we were breastfeeding, so it's a bit different, but not that much).