How do you know?

When did your pediatrician tell you that you should switch to soy? Last week her doctor said she wasnt gaining weight (she's bf) so that I should suppliment with formula. Both breastfeeding (before&after) and with formula she throws most of it up and gets a pretty upset stomach. And it's not a little here and there it's full on when we burp her sometimes projected. We have her on Enfamil gentle and use the playtex bottles because we thought she was getting too much air with the other 5 brands of bottles.