Alahhh I feel for you, honestly lah, that buttock rash from constant pooping is so heartbreaking to watch! Since the clinic's already checked and stool looks normal, it could be the formula, but sometimes at 9 months babies just go through phases like this, so maybe ask your pediatrician specifically about switching to a gentler formula or trying a different brand to see if it helps. Few of my mum friends had this happen and one swore the formula change made a difference, but I've also got friends whose babies just outgrew it after a few weeks, so it's very individual lah. In the meantime, extra nappy changes, letting the area air-dry, and a good barrier cream like Sudocrem really helped their little ones' rashes heal faster