Is the soy formula also gluten free? I'm not very informed about formula but that would be the first thing I would check. If you yourself are diagnosed celiac, your baby could very well be responding to gluten even though he/she may not be celiac. Just keep up your investigation, consitpation in a 3mo old is NOT normal, and don't let any doctor tell you "some babies are just constipated". I won't rant, but you could search for my previous rants if you like. Babies' bodies tell us things, we have to interpret their signals, do NOT ignore this signal. He/she is definately reacting to something.
Thanks. We ended up switching her back to her original formula and everything is fine now. She seems to not be able to digest Enfamil, but Similac is great.