Does anyone have a toddler with GI issues?

My 18 month old daughter, my third child, has been a difficult baby since about 6 weeks. She was colicky (for lack of a better word), slept poorly and was overall not an easygoing baby. This was a stark contrast to my first two children but I know they're all different. Once she hit 10 months old and was still waking up screaming I started to suspect something more was wrong. I talked to my pediatrician about it often but he suggested she may just have a difficult personality. At around 14 months I was really stressed about her frequent night wakings (2-3 times/night and she would just scream unless I held her until she fell back to sleep). It was suggested by some people that it was "behavioral" and that I needed to let her cry it out. I did try that but she would just scream and scream with no let down, plus it was stressful to me to hear her. She has also always had runny/piece-y stools which resulted in severe diaper rash even if I changed it right away. I finally decided to take her off dairy, which I was hesitant to do, but I was desperate. I kept her off dairy for 3 weeks and her stools were somewhat better but still not great. She did seem to be happier though, I figured she may have had less abdominal pain. I put her back on whole milk and the situation got worse again. So now she's off dairy but she still has the bad diapers 75% of the time which are almost like when she was a newborn- not overly smelly but kind of sticky and chunky. We're going to a GI doctor next week to see if there's something going on but I was wondering if anyone else had this issue and what you did to resolve it. I should also mention that my daughter is EXTREMELY picky about what she'll eat. I can probably count on my fingers the amount of food she'll eat but she does get fruits and protein as well as fiber. I'm sure her diet doesn't help but I don't think that's the sole cause of her issues. I'm open to any suggestions or experiences people want to share. Thanks!!