digestion issues!

I'm at a total loss. My baby is formula fed she was on similac sensitive but she wouldn't poo while on it and was uncomfortable because of that so we switched her to neutramigen which had her using the bathroom regularly and we didn't hear her stomach gurgle anymore BUT it made her throw up TONS all the time. So we're trying similac PRO sensitive now and the throwing up def decreased tremendously BUT she's back to not having bowl movements and being uncomfortable.