Hi! I posted this on the formula feeding board, but while I'm not very active on the boards, I have been following September 2014 since I became pregnant and I was hoping someone might be able to help out. Any help would be appreciated

Sorry if this was already asked, I tried to do a quick search but couldn't find the same exact issue. My son is 5 days old (he was born at 41+2 via csection) and we were asked to supplement due to his weight. We've been supplementing since Day 2 with Similac Advanced. He's taken the RTF bottles from the hospital like a champ and was back up to birth weight in no time.
Understanding we will need to continue to supplement until my milk comes in and maybe even beyond that, we purchased the cans of formula due to cost. We started using the powder at around 4pm last night and by around 10pm he was throwing up the formula. He has done nothing more than "drool" or "spit up" very small amounts during a feeding and now he's throwing up what seems to be chunky (assuming powder chunks??).
Does anyone have any suggestions? What am I doing wrong that he is not tolerating the powder but is just fine with the RTF? We're back to RTF for now, but I'm wonder if I should try a different brand or maybe mix it a different way?
Re: Powdered Formula vs Ready to Feed
For my LO formula we used bottled water at room temp because that was what the RTF was at. Maybe trying warming the water before adding the formula since breast milk is mommas temp?
With supplementing newborns it's important to do RTF because their systems are not able to digest the powdered stuff at the newborn phase. Sucks that it's so expensive though