Please help, 6 wk old Preemie cries after eating

My daughter is 6 weeks old and in the past week or so has started screaming/crying after eating but not every feeding. It really seems to vary... usually worse at night. She gets both breast and formula (for fussiness and gas). Has anyone had this problem? Not sure if it's colic or just gas or gerd. She seems like she's in so much pain.....  I've rea that coic should peak at 6 weeks but since she is a preemie, do I have to go by her due date? ANy help woud be greatly appreciated