Oh, I so feel your pain!
DS had a similar problem 3 years ago with his back. He went from an "Ow, my back hurts" to almost full paralysis over some weeks. We went back & from the Docs working up from Paracetamol/Ibuprofen to diclofenac to codeine & finally diazepam.
Everyone said it was muscular and he was on waiting list for an Orthopaedic referral - months away! I took him regularly to an Osteopath, who helped relieve the spasms with massage type techniques, but was worried he had something more severe & eventually would do no more until an MRI had cleared him. He also wrote to our GP. We went to A&E & got nowhere.
DS ended up sleeping on floor on a camping mat as he could not cope on his mattress. He woke in the night & was sent home from school many times.
In the end, school put him on a reduced timetable & I paid for a private appt. The consultant said he could only bend 15 degrees and was worried about benign tumours etc! He got him an NHS MRI scan within the week that he had to be sedated for as he couldn't lie still at all.
Thankfully the scan was clear but he was given weekly hydrotherapy & physio sessions to help him. They decided it had been a severe muscle strain that caused a spasm which took diazepam & hydrotherapy to ease.
The whole thing was a nightmare and did take a few months to resolve.
I sincerely hope your DD gets some answers & help soon.