When would I need to be concerned? feeding and pregnancy?

Um, what? Your youngest is 20 months and you have an older one nursing? I will just let you know that i have memories from being 2 years old and if i was feeding and remember that, it would be a bit traumatic. I think they are both old enough that you can stop to be honest. They dont need it, and if you have weird things happening with your body, i would just stop. I would be worried your "fetus" baby wouldnt be getting enough nutrition if you have 2 babies sucking some out and you are having problems keeping food down. Doesnt sound healthy to the one in the making at all :(