Help - Firm stools

My 3 month old is formula fed (please no judgment, I was unable to breastfeed) and she’s constipated. She is straining a lot to poop and her stools are firm. I gave her some medication from the chemist and this helped but I obviously don’t want to do this long term. We haven’t changed her formula since birth and this has only started in the last week or so. I have an appt booked with her paed but I can’t get in for a few weeks and I don’t want her to be uncomfortable or in pain while we wait. Can anyone recommend any formulas or anything else that would help?