Constipation?

Lindsey • 24 years old married to my best friend. I lost one baby and I'm pregnant now! :)
My baby is 2.5 months old she's both formula and breastfed. She's been constipated she yells when she poops and when she does it's really thick and only a little bit. Is there anything I could give her to make it a little easier? I called her doctor but they won't be able to get back to me till Monday :/