I have to second the Drop-Ins! I tried the others, but the continuous flow are just that: continuous! So, the baby doesn't ever get a chance to catch his/her breath. I also have to say that the LATEX nipples make a huge difference. Both of my kiddos would only every use them. I could only find them at Kmart and online. But, craigslist is a good place to get them. People order extra and sell them still in the package.
If the spit up is only very few days I don't know that it matters much, but you might need to try a different formula. Both of mine had to use the Alimentum, but they were allergic to dairy and soy. My friends' little guy had to have the fussiness formula because the Alimentum made him sick. So, try different things, but if there is any extreme crying, like major tummy upset, then don't wait a few days, just change. Also, you can give a little one Mylanta which helps the acid in the tummy. Give it 1cc or ml at a time and see if that helps. It did for mine.
I hope this helps but a little fussy isn't too bad, a lot is.