Hi pkshl yesterday my 3mos old also had the prob of not pooing...
He hasnt pooed for 36hours and finally pooed today evening.
I did some research and found out that at around 3mos old or gradually when baby gets older. the frequency of poo changes.
If he is on breast milk and FM+plain water shouldnt be constipated..
When he starts on solids then the frequency will reduce even more.
If his abdomen is not bloated, he is not "gasy", he is happy and not straining to pass motion then you have nothin to worry about