Baby throw up/spit up

Nancy
My 18 week old daughter is on Similac Low Iron and has been since she was about a month old, the past few days every time I feed her, I make sure she burps but she always spits up and there's been a few times that there's a lot like she threw up, I'm not sure if it's because she has a bit of a cold right now or if it's the formula??? Any ideas???