Transitioning off Similac Alimentum

[u/No_Cardiologist_6944]

5.5 month old has been on Similac Alimentum since about 10 weeks for severe reflux and sensitive stomach. He did great on it, but reflux and spit up have significantly improved since then so we decided to attempt a transition to Gerber Good Start Gentle Pro. We did a slow transition for about a week, then 100%. Of course he got a cold when we transitioned to 100%. He's been ok for a couple days, but today is day 5 and he's been having some crying episodes and pulling off the bottle. He's also been having what feels like almost constant diarrhea with mucus. I just can't tell if it's the cold or the new formula. He's very congested and coughing but getting better every day. I'm just so worried I have messed things up and his stomach is so upset. I'm not sure how long to stick it out or go back to alimentum. I did send a message to the ped, I just haven't heard back yet. Anyone else go through this?