fed baby not pooping! Help!?

I have been feeding my one month old since birth and he has always been a great pooper, but he has not pooped since yesterday evening. I can tell he is straining a little and he get fussy when he pushes so I know he has to poop it just isn't working out for him. What can I do to help him? I've patted his lower back and put him in his bouncer on vibrate hoping it might help move things a long, but so far, nothing. HELP!