Poo/constipation/lactulose question!

Hi all, Has anyone had any trouble with poops since birth? My little one is on anti reflux milk which is thicker and at first, was causing awful constipation and she was practically unable to poo herself without me helping her (doing bicycle legs and crunches until she pooped), she was then prescribed some Lactulose and I then found we needed to use that every day for her to poo by herself and for it to be soft. She's now 15 weeks tomorrow and I'm just thinking, surely at this age it isn't normal for her to still need help pooing? If I stop using the lactulose even just for one day, you can visibly see how uncomfortable she gets throughout the day and obviously then impacts her naps/night time sleep and everyone just has a horrible time with it. I'm going to mention it again to the HV at her 3-4m check soon but I'm starting to worry she hasn't got strong enough muscles in her bum to poo by herself. Has anyone experienced anything similar? Thanks xxx