HELP!! NEW MOM!!

So my baby is 2 1/2 months old and she's having a hard time pooping, I went to the doctor and they told me to try a little formula once a day. So I been using the WIC formula which is the Similac the blue can and so that hasn't been working for her. I am breastfeeding and using formula but it's been over a week and still hasn't pooped and I am very frustrated because I can see her pushing but I don't know how else to help her, my poor baby just wants to poop. I already scheduled another doctors appointment so we see how that goes but what can I try to help her poop and since I'm breastfeeding what can I eat to help her also? P.s. she's constipated and fussy but the doctor didn't want me to give her water yet.