My five month old keeps throwing up her formula?

On and off for the past couple of weeks, my five month old (almost six), will throw up her formula. It is projectile style, but she also spits up curds a lot in between feedings. She's been on the same formula since she was about two weeks old, and it's lactose sensitive. It always seems to happen when we burp her. She also isn't eating half as much as she was a month ago. But I've been looking for symptoms of dehydration, which she has none...and after she throws up she'll look at you and smile and be happy after wards. I know babies can go through a phase of not eating as much, but this has been at least two weeks. Not sure if she keeps catching something at daycare, or maybe it's reflux. Any ideas??!!