Enfamil AR Causing Constipation?

My daughter is 6weeks old, I had her on the Similac Non GMO Sensitive. But wanted to try something different due to her constantly spitting up. I believe she has Acid Reflux. Well the Enfamil AR has been great all except for one thing. She is extremely constipated. Her bms are so hard and she strains so much go get them out theres blood in her stool. She has a doc appointment tomorrow but I just wanted to know how common this issue is with other moms who use enfamil. And what do you do to help the constipation?