Hi Jenni,
I, too have 2 kids around the same age as yours! (I can understand how busy you are...mine are 2 and 5 months).
My doctor's philosophy is "If they are hungry, feed them".....meaning when they want extra give it, when they are done (even if its less), don't worry about it. For a long time, this caused me frustration---not giving me a direct answer. With my second, I am a lot more relaxed and I've noticed the same patterns my first daughter had come again with my 2nd. My 5 month old is currently eating rice/cereal as well. Too much rice binds her up so I actually put them both in the same serving---mixing 1/2 and 1/2. I do notice that since she's been on the cereal she is taking less of the formula. The cereal fills them up more. Remember, the cereal also contains formula, too.
I've also noticed my baby to eat more (in particular, formula) when she is about to hit a growth spurt. Lastly, another reason I've noticed her eat less from time to time is when her gums are starting to hurt---she is starting to get teeth already. You can tell this when they start gumming the bottle and stuff which you probably already know.
It's great you have a doctor's appointment to make sure nothing else is going on, but rest assured--the same is happening in my home, too.
Good Luck!