Upset stomach since birth

My granddaughter and her mother lives with us. My daughter is in college, so I watch my granddaughter during the day and do the doctors appointments. When she was born my daughter breast fed for the first three months, but my granddaughter would arch, have diarrhea and choke during feedings, so my daughter gave up. She ended up having tongue tie that wasn’t detected until she was six month and was allergic to milk. She was put on so many different formulas because diarrhea just kept up. The doctor said with the last formula it was a side effect from the formula. We also found out she was allergic to eggs, nuts, and tomatoes when she started eating. Since she’s been a year old the doctor has put her on milk and non milk products. He has taken her off of any juice, fruits, and oatmeal. The diarrhea still continued. She has about 4 to 6 diarrhea diapers a day. I was just so sick of them always saying try this or don’t feed her this. I told him at that point we have been dealing with this since shebeen born and there was nothing else to try. I told him I wanted her to see a GI. At the point she was still gaining weight, so he was concerned, but no overly concerned as we was. He did a referral two months ago to a GI, but I was having a hard time getting his staff to send the authorization code over. She had her 18 month checkup a week ago and he wasn’t happy with her weight or growth. She is almost off the bottom of the chart they use. He called his staff into the room and told them he didn’t know what the problem was with getting her into the GI but he wanted it fixed today. She has her appointment tomorrow and I know we won’t get answers tomorrow but it’s a start. Has anybody ever dealt with chronic diarrhea with their child since birth?