2 week old constipation

My baby was born on 9/11. They formula fed her at the hospital because I didn’t produce enough milk. When we got home we got similac to start with. And then my milk came in it wasn’t enough so we would finish it off with some formula. She’s been fussy on and off she only has about one dirty poopy diaper and it’s usually like mustard colored diarrhea kinda. The last 24 hrs she’s been trying to push so hard that her face turns red. And she only had one explosive runny diaper. I’m starting to think she’s lactose and tolerant but I’m not 100%. First time mom and worried. I don’t wanna switch to sensitive formula unless dr says so. What can I do to help my poor baby.