Should i be worried my 5 week old is constipated and in pain ?

Straining, fussing or irregular stooling doesn't mean constipation. The 5 wk breast fed kid may go 7-10 days without a stool or poo a little at each meal. As long as it is soft, i don't consider that constipation. If the kid is putting out dry rabbit pellets or stool bigger than your finger you can sometimes tease a stool out using vasaline

If your newborn appears to be in pain, and especially if there in a change in his/her appetite, it is well worth seeing your pediatrician rather than worry. This is a very new baby.