Not gaining weight needed

My 18 month old daughter as been had trouble since about 4 months old.  She has been through speech therapy for her feeding, she sees a GI specialist for Reflux(which is doing better) she has major constipation and takes 15ml of Lactulose twice a day. My problem is that she is gaining weight slowly, and is still in the not on the chart or at the bottom percent for her age.  About 6 months ago she has had a CF test done, and it came back fine and  had a upper and lower GI done, that came back fine also.

We see her GI doc every 3 months, and last time we saw him he was not to concerned because he said neither me or my husband is big, and we both were born early.

Now last week we took her to her ped doc for her 18 month check up and shots. He seems to think something is wrong with her, and he said we need to run some tests, with or without her GI doc consent.  He put her on Pedisure 8oz three times a day. I went and bought some to give her, and every night since she started drinking it she has been awake all night long crying like her belly hurts. So I stopped giving it to her. Do you think that could have aggravated her reflux? because her reflux has been fine until this, she even stopped taking her reflux medicine 3months ago per GI doc orders.
I just do not know where to turn. She eats all the time, and is very active.

Nicole

[Renee Radenberg]

Her weight is below the 3rd percentile and her height is just below the 25th percentile for her age. If she is gaining weight steadily no matter how slowly then she is fine. If her GI doctor has been treating her and has her off her medication for 6 months (without issues), and with a slow weight gain, then suggest follow his advice. If you want to give her more calories and protein for some weight gain, suggest giving only 4oz of Pediasure 2-3 times a day and see if she tolerates that.