My preemie's belly grows big after feeding?

My baby was born at 34 weeks 2 days. For a week, she was put on tube feeding. After that she began with breast and bottle feeding alternately. A week ago, I brought her back to the NICU due to her big belly (although she burped and pooped regularly). At first, the doctor told us her intestines were immature and couldn't cope with her fast growth--- she was born at 2.2kgs and was 3.3kgs after a month. She was back on tube feeding and IV, but after four days, she was back on bottlefeeding. Today, when I visited her, the doc said she didn't want bottlefeeding again and is back on tube. They can't tell what it's all about and told me they need time to observe. Has this happened to your preemie. Why can't they diagnose? I'm so worried. Please help.