Please help! Out of ideas…

[Dogmum20]

My nearly 9 month old is constantly throwing her head back and arching her back and straightening her legs.
She’s done this since birth but now she’s obviously bigger and stronger and sometimes catches me unawares as she launches her head back out of my arms, with me nearly dropping her.
She’s so miserable especially at nap or bedtimes. And just screams and throws herself backwards while I am rocking her or arches back when she’s lying down and thrashes around the cot until she sits or stands up and just yells.
Even when not a nap time, she won’t sit on my knee to look at something, she straightens her legs after about 10 seconds and arches back and slides off.
She’s quite happy crawling and standing up at the sofa etc.
We’ve been on reflux medication, it didn’t make a difference, I’ve been off dairy, I’ve tried giving milk in different bottles/cups to help with air swallowing (as she’s always swallowing air), no change since starting solids. She’s on laxatives because she doesn’t poo often, but I don’t know if this is the cause of the back arching…?
The doctors weren’t concerned (oh she’ll probably grow out of it, oh it’s probably the constipation, it’ll ease soon…).
Has anyone had anything similar? Could this be reflux, wind, constipation, something else? A combination? I’m just out of ideas and the endless screaming at bedtime and through the night is draining me…. Help!
TIA

I would be looking at other allergy.

My 9month old DD does this and its a pretty recent thing too.
She also suffers from reflux and CMPA which she is medicated for.

I personally find the only time she does it is when she is over tired. If she has had a bottle and im trying to settle her downstairs on my lap she will throw herself about.
As soon as I take her upstairs to my bedroom with the curtains shut, she settles immediately and can be transferred to her cot when asleep enough.
She has also gone through a big developmental leap recently where she has gained teeth, started crawling, pulling to stand and walking with her walker.
She has also learnt that she can get off of the sofa safely by sliding off feet first.
Shes stupidly independant and if she doesnt want to sit on my lap and read a book she just wont.
I cant see the behaviour has been linked to reflux at all but obviously cant rule that out in your case.

I know thats not helpful but for me DD only does it when she is very sleepy. If we dont catch the sleepy cues in time we get the screaming and crying as well.

At 9 month a lot of babies just want to be free/independent and do their own thing, even though they can't quite do it yet.
My little one does this manoeuvre if they don't want to be held.
Some babies just aren't cuddly babies. My little girl had always liked her own space for sleeping and not being cuddled in! Very rarely we had a cuddly sleep past being a few month old.
Since turning around 2-2 and a half she's sat on my knee more and wanted to sit and hug more. Before that there was too much moving and exploring to do.