I have the same problem occasionally. I try to eat enough fibre but it's tough. These protien shakes DO NOT help this problem. You may have a blockage of some sort. Problem is though when you are a young guy and go to a doctor about your guts, the usual response is "Your young, you shouldn't have anything to worry about til your in your 60's". Then of course about a week later your in emergency cause your doubled up in pain and your slowly being poisoned by your own bowels.
Try taking a dose or two (1/2 to a whole bottle) of "Fleet" oral laxative. That should clean you out good. Remember though, STAY AT HOME for 6 or 7 hours after you take this stuff and camp out in the toilet. It works very well (despite tasting like absolute crap!). Taking fibre AFTER you are constapated can sometimes exacerbate the problem (sometimes it does work though too).
As for diet, make sure you are eating something like a bowl of all bran a day PLUS lots of veggies (not just greens but other stuff too) and some fucking fruit too. No fruit=no shit.
I just went through 5 days of hell with the exact same problem except I wasn't "giving birth" til I did the fleet (they have an enema too but that would be a LAST resort).
Good luck 2U. Like to hear what your doctor tells you (my doctor is an IDIOT)