Kristin,
First off, I'm sorry for what you are going through. I know things like this can be scary. I wanted to say that I searched for a Pediatric GI about a month ago for my 5 year old daughter and could not find anyone near me who would see a child under the age of 6. The direction I was pointed in was Children's Hospital. In our case we went to San Diego but ended up not having to see the specialist. You could start there, call the Children's hospital in your area and make an appointment or have you tried asking your Ped. for a referral? I assume your Ped told you she isn't retaining water in her intestines? Do we know this for sure? My daughter suffered from chronic/extreme constipation up until a month ago, and it started around the age of 15 mos or so, this too was the time she started solids. Long story short - I didn't catch the initial constipation, this lead to very large/hard and painful bowel movements, because of this, she started the vicious cycle of holding her poop in and we've been on a crazy ride until last month! We ended up seeing a Pediatric Specialist (not a GI specialist) in Murrieta, who seems to know a lot about the GI (Her hubby is a GI Specialist but doesn't see kids). Sorry - long way of telling you to stay on top of it and get help now so it doesn't lead to what my daughter went through.
Good luck,
M.