Nothing wrong per se. You could have eaten something lo is sensitive to, like spicy food. Or, you could be over feeding bm. Babies need less bm in oz than they do formula.
There are a ton of reasons for spit up and a lot of them have nothing to do with whether it is formula or breast milk. Just watch and see if you find a pattern that satisfy suggests it is the breast milk. It could have just been a fluke too and might not have anything to do with anything!
I ebf and dd spits up multiple times after every feeding. She gained 3 lbs I the first month so she gets plenty. I'm eliminating dairy to see if that helps. On day 3. I hear it can take 2-3 weeks to leave my system so I'm giving it time.
Re: Spit up
We're one and done!