For the last day or so my baby's poop has been really runny and dark... He's been eating baby food…

For the last day or so my baby's poop has been really runny and dark... He's been eating baby food and oatmeal cereal for about a month now and this just started within the week! My mother in law said he could be teething and that it could cause diarrhea... Any moms that could give me some insight on this?!