What Causes Mucus In Stools Of An Infant?

Hi, I have a four month old baby girl, she is formula fed and usually poops just once a day but for the last few days has been going once or twice after each bottle, the consistancy is odd, often with mucus in and they were smelling quite vinegary. I put it down to teething because she has been chewing a lot but today her poop smells fishy, is this normal for a teething baby or could it be something else? She seems absolutely fine in herself and is as happy as ever.