Newborn not pooping

My little one is 6 weeks old. For the first 2-3 weeks she pooped a few times a day. Then she just quit and became very fussy and colic like and passes really stinky gas but no poop. Her belly was very bloated. So after day 10 of no poop I gave her a suppository and she pooped alot but it was all soft. Then she went 7 days same symptoms and no poop so I did another suppositoriy and again lots of poop but soft. I decided maybe it was a milk allergy so I switched her from breastmilk to soy formula. She is pooping now because of the formula but is still super gassy and colic like and fussy. I can't afford formula for ever so my question is should I change my diet and cut out dairy or could it be something else that is bothering her. Maybe iron from my prenatal vitamin or her being preemie she was born at 35 weeks. Any thoughts appreciated.