Calling all formula moms !!!

What formula are you feeding your babies ?!!! My son is 2 weeks old and is always crying after eating , he didn't poo for three days so I took him to the doctors , she said Vaseline my pinky finger or qtip and put the tip in him bum to help move things along - Kay great that worked ... But he can't pooh on his own without me doing that he just screams and cries I just changed him from iron fortified similac food to goodstart by nestle in hopes that his bowels start moving on their own yesterday ... However , he's still in lots of pain from gas ! I know it will take a few days to fix the iron levels so he can pooh properly but my poor guy is in so much pain! Anyone else experience this ?? And what formula do you use ??