Hi Claire,
It sounds to me like it’s probably just the cereal. If she doesn’t seem uncomfortable, and since she’s still going a couple of times a day, I wouldn’t worry about anything.
We used to thicken Bryce’s breast milk with cereal. I know it doesn’t actually get thick with breast milk, but it seemed to work anyway with Bryce, and helped his vomiting a little. I guess once it’s in the stomach it weighs things down. We had to alternate rice and oatmeal – otherwise Bryce got a little constipated.