21 days prematurely delivered child, birth at 2 kg weight, has normal growth for 3 moths, and was taking feeds as per chart, but always cries with each feed if more is given as per the chart. But now takes only 600 cc per day (expressed bottle feed). If tried to feed more, gets Colics and vomits sometimes. What should be practiced?

(only demand feed or any other investigation is needed). As per today the weight gain is normal 4.5 kg in 3 months. How come the weight gain is normal?