it's likely due to the fact that a Moby is just not supportive enough for a child of that size, thus it's putting a lot of your child's weight onto the knot, which is then pushing into your stomach. have you considered a different carrier? honestly i can't imagine even trying to carry a 2 yo in a front carry in a stretchy wrap - it sounds seriously uncomfortable. i don't like stretchy wraps once a baby hits about 15 pounds, myself. plus i'm not a big fan of front carries. i can barely see over the head of 10 month old in a front carry, i can't imagine trying to see over a 2 yo!
you should try a back carry with maybe a SSC (like an Ergo) or a Mei Tai (Kozy is good for bigger kids). my inclination is that you'd be better off with a SSC because the buckle is lower so you won't have anything pushing on your belly at all.
if you really want to stick with a front carry and a wrap, you need a good sturdy woven wrap that won't sag. and one long enough that you can tie it a little off-center and thus have a knot off to the side. though your child's weight might be enough pressure on your stomach without the knot, so i would think your best bet is to get comfortable doing a back carry (don't try that in the Moby, btw - stretchy wraps are not safe for back carries).
