Initially I thought the question was more general so I'm going with my first thoughts in case anyone is interested.
She was induced at 38wks (artificial rupture only required) due to IUGR. She weighed 5lbs10 and measured only about 18" (46cm). Over the first few days of her life she had her heart and kidneys scanned (no congenital defects) and numerous blood tests to confirm her diagnosis and check her jaundice, blood counts etc. She was pretty sleepy so I had to express and try to syringe feed her. At 3 days old we were able to come home and she has feed fine since. She had her hearing checked when she was only a few weeks old via auditory brain response and a couple of other tests and that was fine. She has TINY tubes, including ear canal, noone can see the ear drum on one side so it is tricky to check for infections. She is doing really well - her only issues are this stomach pain and pretty persistant upper respiratory infections, not sure if it is due to anatomical, immunity or combo (any advice welcome!).

Respiratory issues are a very common issue in preeemies due to underdevelopment of the lungs.

She is delayed in reaching her physical milestones due to hypotonia (floppy, low muscle tone) but is pretty much sitting by herself and very close to crawling.
In fact, most of her development happens in focused spurts and, although it may just be coincidence, since starting oat bran, rice bran, kelp and lecithin, she made really good gains in her speech, muscle tone and her constipation has more or less resolved simultaneously!? acetylcholine?

Acetylcholine is part of the answer. Lecithin also contains phospholipids that also help with the development of the brain and other parts of the nervous system. Rice bran and kelp provide silica and B vitamins also required for proper nervous system development and function. I highly recommend also adding the trimethylglycine (TMG), which I have seen work wonders for autistic children. I know she is not autistic but they share some of the same issues.

As for the stomach issue this still sounds like a flora issue. Especially with the syringe feeding, which would also have reduced her exposure to the good flora from breast feeding.  To make matters worse the time she spent in the hospital could have exposed her to a number of pathogens that she would have been more susceptible to with the lack of flora.