We had the same problem around the 3 week mark Nic and it was just a matter of hanging in there. It only lasted 3 days though so I'm sure it's worse for you after everything else. I totally sumpathise with you, I feel like a failure when I can't work out what the crying is for, it's definately not an easy road for any of us so far.
Can I ask what nappies everyone has been using and their thoughts?
Are you breast feeding Nic ( sorry I can't remember!)