3 week old not pooping?!

When baby was drinking breastmilk she would poop several times a day . Her poop was mustard and a buttery consistency but now that i been giving her similac she hardly ever poops and when she does is literally one dark green solid/mushy ball and thats once maybe in 2 days or more at a time. Im gonna give her doc a ring but wanted some advice on what i can do to get her going ? Thanks