My 2 month old baby is having watery stool with lots of farts and sour smell. Since sunday morning but only 3 big stools and lots of smear kind.

Infant diarrhea. It sounds like your infant may have gastroenteritis, an intestinal infection. Most cases are viral, but, if there is blood in the stool, this could indicate either a bacterial infection or a milk protein allergy. Continued watery stools can quickly lead to dehydration in infants. You should definitely have the baby seen by the pediatrician as soon as possible.