Re: suddenly spitting up
Welcome to the forum and congratulations on the new baby!
Spit-up that is produced without evidence of pain (screaming, back-arching) in a baby who is gaining weight at a normal rate is a laundry issue, not a health issue. Spit-up happens because the muscle sphincters which keep stomach contents down where they belong are weak in very young babies, but like all baby's other muscles, they strengthen with time. Spit-up is likely to be a part of your life for at least a few months, but it usually calms down a lot as the first year progresses. Best things to do for spit-up: feed on demand, be patient, carry spare outfits and burp cloths in your diaper bag.
So why is baby spitting up now, and not for the last 3 weeks? It may be because milk supply often starts to peak at around 3 weeks, and a tummy that is full to the brim gives up a measure of spit-up more easily. If you feel like you have a lot of milk- maybe you feel full or engorged a lot, you're leaking a lot, feeling strong letdowns, baby is pulling off the breast during feedings, you sometimes see milk squirt or stream from the breast- then let us know. There are some things you can do about that, starting with adopting reclined nursing positions. (Reclined positions enlist gravity to slow the milk flow to baby, and slower feedings can result in less spit-up.)
There's a big growth spurt at around 3 weeks, and that could account for the shorter sleep stretches and more difficulty in settling. Do you have a sling? By 3 weeks, many babies have discovered that they don't like being put down, and they will fuss until someone picks them up again. Sling-riding babies are often more content in between feedings. Also, an upright position (like you'd see in a Baby Bjorn or Moby wrap) can help tummy contents stay down, thanks to gravity.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"