All Tests are Negative. Doctor is saying it's Myositis

Mayo Profile - Negative
ANA Profile - Negative
Biopsy - Negative
CPK was varying.

The doctor is saying it's inflammatory myositis because all symptoms are the same and the patient is reacting to the medication.
Medication:-
Wysolone 50mg
Shelcal 500mg

My wife was suffering from tiredness in her leg and couldn't stand, walk, get up from a sitting position, etc...

First, it was very mild it's not noticeable at all. it was like laziness, always feeling like sitting or laying down. we consulted a doc on 2nd March 2022 as she was not having her periods for a month.

After the medication next month it was around 15 days of continuous flow for next month. Based on the doctor's recommendation we have done a PCOS test and the result was positive. The doctor advised doing exercise and diet changes.

Then we noticed heavy tiredness and leg pain while walking We consulted a general physician on 11th July 2022 as tiredness and leg pain increased. started taking Vitamin D supplements as it was low. In a month Vitamin D become normal and Vitamin B12 becomes low tested on the 1st of September. We started taking supplements. Later B12 got normal but leg pain was the same.

We consulted a neuro, and based on the physical examination he said everything seems normal and suggested a CPK test. Which was around 445 first time and he suggested taking vitamin tablets for the next 2 weeks. But in a week she started getting pain in her facial muscles and show couldn't chew or talk. We tested CPK and it was around 2000. We went to a Rheumatologist. Based on the physical exam she mentioned it looks like "inflammatory myositis". We got admitted and started diagnosing. MRI. CPK went high as 6000+ (2 days after testing 2000).

And the doctor started steroids and she was getting improved. After 10 days we have done CPK and it got reduced to 600 and the ANA profile was also negative. How do we confirm it's inflammatory myositis? Currently, she is taking steroids. How do we stop steroids and find out what is actual reason?

CPK History:-
01-09-2022 445
04-09-2022 345 CK-MB 17.46
11-09-2022 1956 CK-MB 0.5
14-09-2022 6445
26-09-2022 690

From discharge summary:

"RNST was not s/o any decremental response while NCS revealed mild left ulnar axonal neuropathy.
1/V/O high CPK levels she was started on inj.dexa on 14/9/22 and muscle biopsy was performed on
15/9/22. Patient has been currently started on steroids and methotrexate since MRI was s/o
diagnosis while myositis panel has been awaited. She is currently being discharged in
hemodynamically stable condition to follow up in rheumatology OPD after 10 days."

First follow up after 10 days:-

Muscle biopsy - negative for IIM
Myositis profile - negative
Overall clinical features are c/w inflamatory myositis including treatment response

Background history:
h/c paresethesia% in 3/L lower limbs in 2017: evaluated and found to have
Vitamin 312 DIFICIENCY- TREATED
2021: COVID infection during 7th month of gestation- underwent emergency LSCS during the same. I/v/o oligohydramnios

Family history:-
cerebral palsy in elder sister(delayed crying during delivery)
muscular weakness in 1st cousin (lower limb weakness* respiratory insufficiency)