This is something babies do. What looks like her entire meal is more likely a lot of drool and mucus, some stomach acid, and just enough breast milk to tint it white. There are some situations where a large amount of milk/formula gets burped up, and this is usually one of two situations; an overfull stomach (some babies will eat to bursting; the reflexes involved may allow for nothing else) or air behind a large amount of milk/formula (usually happens when feeding a baby on their back or side).
Babies, especially brand new ones, don't have all their systems working perfectly in concert yet. Over the coming weeks you may notice she may not poop for a day or even two, though she's eating plenty, and then she'll have a massive blowout. That's her peristalsis movements trying to get in gear in fits and starts. The stomach is similar; she'll hold formula for a while without really processing it, then she'll get a burst of stomach acids that will cause heartburn, belching and spit-up. It will all calm down and get into rhythm within the first couple of months.
As long as your baby is gaining weight and appears satisfied after feedings, you're just fine. Just keep a receiving blanket or burp cloth handy to put over your shoulder.
Things you can do to reduce spit-up include:
- More frequent burping; if you normally switch breasts in the middle of a feeding, burp her when switching.
- Feeding in a more upright position; instead of the cross-body hold or the football, try reclining about 30-40 degrees and then laying her along your stomach, on hers.
- Making sure she's got a good latch and is not sucking air as she feeds.
- Watching what you eat; the baby eats what you eat, so compounds in your food that cause belching and gas for you will show up in your breast milk and cause belching and gas for your baby too.
- If you were formula feeding I would have recommended trying a formula designed for sensitive stomachs and/or spit-up reduction. These contain proteins that are more broken down than traditional milk-based products, and rice starches to make the formula thicker and less likely to be regurgitated. They work; my wife wasn't able to breastfeed and so we've been bottle-feeding a mix of pumped breastmilk and formula, and have noticed when we use the "for fussines and gas" and "spit-up" formulations it really does help.