My baby tends to do this when she eats too much. Sometimes she falls into a light sleep on the boob, but keeps flutter-sucking/playing with the nipple and thus I keep having let-downs, which she sleepily drinks, ignoring the fact that she is full, because she's sleepy. Then she wakes up and grizzles with what I assume must be stomach ache. Four times out of five she will then projectile-spit everything, before returning to her usual cheerful self. She suffers a little with mild reflux, mostly the silent kind where the milk comes up but doesn't usually come out, and this tends to be worst at the end of a day full of feeds.
Your baby may be over-filling herself by cluster feeding, or swallowing a lot of air while feeding which then brings up the milk when it comes out. You can try to remedy this if the spit-up bothers you, but if it's not bothering your baby it's likely just that - harmless spit-up. Actual vomiting usually causes a baby to become very upset and shocked.
If she's gaining well, don't worry too much, but if it happens more frequently it could be a sign of pyloric stenosis which requires surgery.
Most babies do projectile-spit though at some point, and my daughter goes through refluxy and not-so-refluxy stages. I think you're fine. Doctors will help though if you're worried.
