It might be worth a call to your OB, just to check things out. By 6 weeks things should be back to normal.
I didn't experience this myself, but I've heard several women make similar comments. Pregnancy and breastfeeding both affect your hormones so much I would think it could just be something like that. An overload of hormones can definitely make you feel nauseous, and it takes a while for them to regulate when you're breastfeeding. I would definitely mention it to your doctor just to be safe, but try not to worry too much. It's probably just another one of those wacky things we get to enjoy as women :)
Is it just while you are nursing or a constant cramping? Also is it up high or lower in the abdomen? I would definitely check things out with your doctor. I was having a similar situation, and my pain turned out to be gallstones, which apparently are common in pregnancy. I ended up having to have a scope to remove stones from both my liver and pancreas, and surgery to remove my gallbladder. I hope that everything turns out ok for you.