Re: Oversupply or some other issue?
Welcome to the forum! What you describe could definitely be oversupply. You can have oversupply with small breasts because breast size is mostly about fat, not about glandular tissue. Big breasts have more fat, not more milk-producing infrastructure. There are small-breasted moms with oversupply, and large-breasted moms who struggle to get enough- in short, size doesn't matter.
That being said, you're not feeling "overly" full or "distended", so if you have an oversupply issue it's probably on its way out. Most moms start out with some degree of oversupply; it's nature's way of making sure a baby gets fed while mastering the art of nursing. But as time goes on, the body detects that it is overproducing and adjusts supply to meet demand very precisely. This adjustment is desirable because making too much milk is a waste of energy and puts a mom at increased risk for plugged ducts and mastitis.
It sounds like you may still have some degree of oversupply because of the painful letdowns (more milk usually means a stronger letdown sensation) and because baby seems to prefer a shallower latch (babies often learn to use a shallow latch or compress the nipple in order to slow milk flow). And if you're still feeling occasionally full, that would point to oversupply. You don't need too much extra milk to be making too much- remember that the average feeding is just 2-4 oz (that's true throughout the first year, BTW).
Green poops can have causes aside from oversupply. Illness, teething, supplements, insufficient milk intake, and allergies can all explain green poops. But none of those sound like particularly good explanations for your baby. He's likely too young for teething, you don't mention any supplements, his weight gain is good so it's not insufficient milk intake, and you've eliminated the top allergens so it's unlikely to be allergies. So my money's on oversupply.
In your shoes, I would take a watchful approach to this issue. As long as your baby's weight gain remains normal- and please note that even with oversupply you will reach a point at which his weight gain will slow and he may drop percentiles on the chart- and his health is generally good, green poops are not a health problem. They're more of a variation on normal. Fussiness is normal for all young babies, and the "frequent" feedings you are experiencing are totally normal. Breastmilk digests in just 90 minutes or so, which means that most babies nurse every 1-3 hours, or even more frequently.
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