Why is my baby spitting up

Katryce
We use dr brown bottles, and breastfeed. I dont think she has gas, because she ALWAYS has a good burp after feedings. She isnt nursing long enough to make me think she over ate. Shes not keeping any of her milk down. Super lethargic, no fever that i can tell of. Normal poops. This has been over the last 2 days. Any guesses.?