She sounds exactly the same as my 6 week old DD. Also as my DS was at that age. Im afraid to say they both have/had reflux.
It might not be, it might just be colic.
Look for other signs though. Does she hate being lay flat?
Does she have to be held upright after a feed or she is sick? If not it could still be silent reflux when they are not sick.
Is she snuffley? Coughing a little as if something in her throat?
Listen closley when she comes off and is frantic, is she gargling liquid in her throat, almost choking it back?
She might not be too obvious at 3 weeks but keep an eye out.
Colic and wind problems come hand in hand with reflux so the following will help with both incase she doesn't have reflux and just has colic.
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tilt cot/moses basket 30 degrees (i use a plastic storage box but you can use books. It's quite a high tilt.
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Once she has calmed down and dosing off on you, put a hairdryer on for the soothing white noise, lay her tummy first, head to side in her basket/cot. The hairdryer will keep her dosing and stop her fighting the new position at first. If your uneasy with this in the night, do it during the day when your keeping an eye out. Babies break wind in this position and it soothes their tummies. She will sleep sounder and longer as the startle reflex wont wake her. Then her tummy will cope better for next feed.
It will also make sure the feeds are more spaced out as she needs at least 2 hours, ideally 3 to digest a full feed. One feed on top of the other causes colic.
If you find you'd like to sleep her on her tummy at night to get the extra sleep but are nervous - buy a moniter such as tommee tipee with sensor pad. It has a flashing light every breath and an alarm sounds if they havent taken a breath in 20 seconds - peace of mind.
If she does have reflux the doctor will give you gaviscon. If it's mild it will help, if its bad only omeprazple and domperidon will help.
It lasts for up to a year but is easier if you know what it is and how to treat.
I was trying to manage it on my own but she is getting worse so have to get stronger meds.
Its the feeding I find impossible too. On off screaming, frantic feeding.
I hope your DD just has colic, good luck. x