Aah, memang biasa lah when babies switch milk and start solids, the tummy needs time to adjust! Since dia baru switch from lactose-free to normal milk, that lactose content might be harder for baby to digest right now, especially combined with solids. Kawan saya punya baby juga sempat constipated masa transition, so try giving more water (if baby is ready), encourage more fruits like prunes or pear puree, and massage the tummy gently, tapi sebab this involves the milk change, better check with your paediatrician to make sure the new milk is suiting baby okay or if you need to switch back or go slower with the transition. How old is your little one?