My son has this issue. At 13mths he got chicken pox on his bottom and I guess inside and found passing poos very painful. The pain eased but 10mths on he still hates doing it. He's always had pooing issues right from the beginning and normally just had one poo a day, but since dropping milk he is going every 2-3 days and the poos have become firmer, which makes him more anxious. When he went 6 days without a bm we did get movicol but I don't feel his is normal constipation. It's more anxiety about going. He also prefers to go out of sight, doesn't like going on the potty (he wants to stand) and sometimes wants our reassurance, so I cuddle and stroke his back. If you can find something to loosen them naturally (we use blueberries), it can make the movements easier to pass. As long as you know their hydrated and eating a balanced diet, then just offer reassurance.