SLTD – congrats on the new baby!
A few thoughts-
1) If baby is gaining well, urinating and pooping frequently (usually after most feeds at this age), and seems satisfied for at least an hour (ideally 2-ish) after nursing, then he is likely eating plenty.
2) Babies around 3 weeks can nurse frequently as they go through a growth spurt around the 1 month mark – this increase in nursing behavior will usually last a couple of days.
3) Reflux – some babies with serious reflux (GERD) begin to show early signs around a month old. Reflux babies will often cry a lot, and show hunger cues (rooting, desire to suck/nurse/feed constantly), because of their GI distress/discomfort. Reflux babies – even breastfed ones- may “overeat” because the action of eating and consuming soothes their stomach for a time. Remember, babies only have a very few limited ways of communicating uncomfortable tummy sensations like gas, hunger, or reflux pain – crying, scrunching/moving their body, and hunger cues (rooting, sucking, mouth movements). The difficulty is knowing whether the baby is acting hungry because they really need to eat, or if they are acting hungry because they are uncomfortable in some other way. Either way, it is always best to offer food just in case as underfeeding an infant is way more harmful than giving them a little too much.
More on reflux - Most babies have a little bit of reflux which is considered normal spitting after a meal (a little that comes up, after some feeds but not always). In typical babies, the spit up is often due to burps/wind and the natural "back up" of fluid that can come up from their stomachs as the valve isn't fully developed yet. But problematic reflux known as GERD is diagnosed in those babies who have persistent pain/discomfort, high volumes of spit up (soaking through rags, projectile spits), frequent spit up (multiple spits, usually after every feed), and substandard weight gain (because they lose so much nutrition while spitting up or because they get too fussy to actually consume sufficient quantities even though they might be on and off the breast or bottle a lot). They may frequently spit up during feeds, before they even finish the meal, or even choke and gag during meals because the food is coming back up as they are trying to drink. Both my girls had classic GERD. My second one began showing signs of it around 2-3 weeks, with frequent spitting, constant rooting/acting hungry, and starting to cry shortly after meals. By 5 weeks old, her weight gain was less than ideal and she was rapidly dropping percentiles as her volume of spit up dramatically increased. By 8 weeks she was screaming in pain for hours every evening and was unusually difficult to settle. It often gets progressively worse over the course of the first three or four months of life before it takes a turn for the better and starts to improve. If you notice the warning signs of GERD (pain, high volume spit up, very frequent spit up, difficulty feeding), be sure to mention them to the baby’s pediatrician. Most GERD is effectively treated with non-medical interventions like keeping the baby upright after feeds, taking the time to burp/wind the baby frequently during feeds, and thickening formula or breastmilk if spit up volume is a big issue. But if a baby is still in serious pain, and if that pain is affecting feeding and resulting in poor weight gain, sometimes medications are prescribed. The good news is, most babies with GERD do just fine and grow out of it by 6 months. With both of mine, it was like someone flipped a switch at the 6 month mark. They went from super soakers to completely normal baby spit up and no more discomfort.
PS: I AM SO, SO SO HAPPY YOU FED YOUR BABY! What you describe – the lethargy and the screaming – is a classic indicator of an underfed child. Great job identifying what your baby needed. You should be really proud of yourself for making the right call under such difficult circumstances (new parent, early postpartum, sleep deprived).