What Causes Constipation And Smelly Stools In Babies?

Hi.. I have an 11 week old baby boy since birth only poos every 2 to 3 days occassionally forth day although i have checked with my dr and a sample has been checked and nothing appears to be wrong but he always seems to be constipated ...he has formula milk but i have noticed last couple of poos have smelt like vinegar although the poop appears to look normal yellowy colour .... why does it smell like this?