Need some advice

So I breastfeed mostly (we do add in a formula bottle here or there and my doctor actually suggested supplementing sometimes) and recently my now 6 week old poops have changed from the graining texture that u get with breastfed babies to almost a slimy texture. This was after a day of basically no poops (he was still peeing and eating normally but seemed gassy compared to normal) and before that he was pooping less at a time but I thought nothing of it cus my oldest had this happen as a baby same with my middle child. Well I was notified by the app that it could be signs of something up and I plan on calling my son's doctor tomorrow but im wondering if anyone else has had this happen.