When I saw your post, I had to respond. Two years ago, I was in your shoes. I was pregnant and due Dec. 15th. I was not feeling well and unable to gain any weight for the first 14 weeks of pregnancy. I had terrible pain and D every day and the doc didn't really know what to say. Here is what I did:
First I started to follow my cravings. I found that I could no longer eat soy cheese, but I could eat real cheese without problems. I practically lived on Pop Tarts the 1st trimester. I'm not saying this was good really, but at least they didn't give me D! I would emphasize to go slow in introducing foods, but try to eat what you crave. It worked for me and I had many food phobias at the time, but the drive to eat was so strong that I overcame it somehow.
I switched from Citrucel to Benefiber during my 10th week and the D stopped a couple of days after that. I felt sooooo much better on the benefiber. I couldn't believe the difference.
Hang in there. I know you are really excited and it's all worth the pain, but hopefully these tips will give you some relief sooner than later!