poo is smelly

breastfed poo actually smells sweet. Not sure what you mean by smelly. If there is an infection it would smell of rotten eggs or really vile, there will be no mistaking that.
As far as straining to go goes, yes some children are more aware of their bodily processes and do tend to make faces... I would not really worry if there is no fever, loss of appetite, vomiting. I have noticed that my LO tends to have those symptoms before he actually has diarrhoea, even when he was teeny tiny.

 

Mine is 1.5 months old and still strains. I was told by the paed that this is normal. However, straining is much less now when compared to when she was ur son's age.