Evan has been horribly miserable for the last week. We started him on some Enfelac formula just to top him up before bed, and that seems to have backfired. He appears to have reacted badly to the formula - no bowel movements for a week, followed by an explosive (less than a second long, diaper exploding, ick...) one.
Good news is, he seemed a bit happier after, well, relieving himself...
This, of course, left us wondering what the heck was going on. Turns out there is a condition called "malabsorbed carbohydrate" where the little digestive system can't fully process carbohydrates in the upper tract (stomach and small intestine), so they get passed to the lower tract (large intestine), where they ferment and produce a lot of gas. This explains the incessant farting, and apparent pain, followed by the mother-of-all-diaper-fillings.
We're going to try switching back to 100% breastfeeding, and see if that makes a difference. He's due for his 6 week checkup on Thursday, so hopefully everything will be resolved by then.