I would have suggested a cold, if you hadn't said that she didn't seem sick. Riv swallows his mucous, which affects his poo, and fills his tummy so he's not as hungry sometimes. The mucous didn't really make him spew, but I'm sure it could cause other babies to.
A cold *might* be harder to notice in this weather since it's warm...

That's the only suggestion I have, but if you're worried Amy, take her to a dr - then at least you'll have some peace of mind