With the huge vomitting only having happened once, it would be very hard to diagnose any cause/pattern. It's fairly common for babies to spit up small amounts of milk, especially if lying down straight after a feed, as the muscles that keep food in their tummy are underdeveloped. It is possible that most recently she just took on too much milk and had to get rid of it. This is especially likely if you have a fast flow, plentiful milk supply and/or it was a while since her previous feed so you were engorged. This happening occasionally while your supply settles down is not necessarily going to be anything more than an inconvenience. If however it starts to happen multiple times a day (full feeds coming up nearly every feed), it could cause her to need to feed more and more frequently to make up for milk she has lost (therefore increasing your supply and potentially making vomitting worse) or she could start not to put on enough weight.
Positions that slow the flow of milk and give her more control over unlatching herself if she is overwhelmed can help (more details can be given here if you do not have breastfeeding support nearby to show you some of these, but I don't want to overwhelm you unnecessarily). Feeding with her in a more upright position and keeping her upright after feeds could also help, but again it's a lot to get your head round if it's not necessary - so maybe wait and see if a pattern emerges? Ensuring the best latch you can also prevents a lot of air swallowing that can cause spit up, as small bits of milk come up with the burp.
Or it could be just a one off isolated thing, or a sickness germ.
