Diarrhoea?

Oh don't worry too much, a few of us in my mum group went through the same confusion at around 3 months! For mixed-fed babies, the poop can already vary quite a bit since breastmilk tends to produce looser, more frequent stools while formula makes it thicker and more yellowish-brown, so the combination can make things look inconsistent from day to day. The key thing to watch is whether the poop is very watery and coming out in larger volumes than usual, if it's just a bit more frequent but still smallish amounts, it may not be true diarrhoea. That said, do keep a close eye on whether he seems unsettled, is feeding well, and has enough wet diapers, and it's always worth checking with your paediatrician just to be sure!