My 2wk old baby only poops once a day or sometimes once every 2 or 3 days should i be concerned?

No Concern: It is perfectly normal for a two week old to have bowel movements this way. No cause for concern. :)

Breast or formula ?: Breast fed infants can go a little every feed or overflow a diaper once a week.Breast milk is so well digested it may not leave enough bulk to need to poop. If it is soft when they go i don't worry about it. Formula fed babies usually have a more predictable pattern of daily stools.

Yes: You need to talk to your pediatrician about this. Sometimes milk allergy can present as constipation but there may be other issues as well.

Everything's allrigh: Infants are all different and stooling depends upon many factors including those that we do not control. As long as baby does not get distended or vomit every feed there is no need to worry. Especially if infant feeds well. Please make sure to visit Pediatrician in scheduled appointments. Don't think you need to worry for now.